I Know that you have finished your study in tropical medicine and hygiene at this school. And so, surely, I am not wide of the mark if I suppose there is very little in the way of facts and views you have not, at least, heard of. I think it rather useless to try to add one more small fact, or one more insignificant view, to the hoard of knowledge stored in your brains, or in your notebooks.

In the present lecture I shall have to go back to the very beginning of modern knowledge on malaria. I shall incur grave risks of telling you things you know already; as a matter of fact, it is more than a risk, it is a certainty. But I cannot help myself. All I can do is to pass over the initial stages as summarily as possible.

The behaviour of wild rats towards an unfamiliar food presents problems of both practical and theoretical interest (Chitty, 1950; Shorten, 1950; Thompson, 1948). When first eaten the food is often taken in very small amounts; if poison is present its effects soon make the rats stop feeding, and if they recover the rats are often found to be bait-shy; that is, they refuse the bait entirely (Rzoska, 1950). As far as rat control is concerned prebaiting with harmless food can overcome this difficulty by inducing rats to feed rapidly on the bait when poison is eventually added (see Barnett, 1948, for a review); and if there are even then a few bait-shy survivors they may be disposed of by a second prebaiting and poisoning with different materials. In large-scale practice, however, this elaborate programme presents considerable difficulties for organizations such as local health authorities, and they are often unable to adhere to it.

The appearance of vitamin E deficiency in rats and mice after the introduction of 2% of cod-liver oil to a diet containing 85% of whole-grain ground cereals is described. Evidence is given to show that the deficiency can be prevented by the reduction of the cod-liver oil content to 1%.

An account is given of the behaviour of two porcine field strains of the virus of foot-and-mouth disease, with evidence of their adaptation to swine, based on experiments on infection by contact.

Thirty strains of Pseudomonas aeruginosa, isolated from routine clinical material, were investigated, with particular reference to the metallic lytic effect and the bacteriophages they carry. From the filtrates of these strains, twenty-five bacteriophages or phage variants were isolated, and the effect of these on the various bacterial strains and some of their variants were investigated.

It has not been possible to elucidate the nature of the agent responsible for the metallic lytic effect. The true phages isolated, however, present many features of interest which warrant their more detailed study.

Part of this work was carried out in the Nkana Kitwe and C.S.I.R. virus research unit at the U.C.T. Pathology Department.

A paper on biological assays and biological standards might be expected to start with a definition. What is a biological assay and what is a biological standard? However, I shall for the moment make the somewhat doubtful assumption that we all know the answer to those questions but shall return to them later, making a start meanwhile by discussing some aspects of the history of our subject.

1. The paper represents an attempt to estimate the error of the standard 10 ml. Haldane gas analysis apparatus under routine conditions of use.

2. The ultimate error of the instrument under optimum conditions has been obtained from duplicate mercury calibrations of the same instrument and corresponds to s.d. = 0·013% gas. A significant day-to-day variation in calibration was found.

3. The accuracy in routine use has been determined by replicate estimations of CO2 and O2 in air and from estimation of CO2 in designed experiments. The data show an error corresponding to s.d. = 0·027 to 0·13%. This is far larger than that described by Haldane.

4. Examination of the distribution of terminal digits in burette readings shows that the smallest scale division is in fact not being divided into ten parts by the worker. Preference for certain digits varies with the individual, with time, and with the accuracy required.

5. There is a marked tendency for a worker to obtain an ‘expected result’, whether this is derived from a previous estimation of the same sample or from knowledge of the expected result; such bias tends to reduce the apparent error of estimation. Bias of replication—‘prejudice error’—may play a part in many forms of measurement.

6. The results and conclusions are discussed in relation to other published data on errors of measurement in general.