1. The development of Theileria parva in engorged larvae of R. appendiculatus exposed to a temperature of 4–6° C. for 3 days at intervals of 2, 4, 6, 8 and 9 days after repletion is not retarded; and it appears, from previous experiments and those now described, that, as long as the tick survives, climatic conditions do not kill or weaken the parasite.

2. The virulence of East Coast fever transmitted by ticks fed in the tail brush did not differ from the disease conveyed by ticks infesting the ears.

3. Reference is made to records of mild reactions to Bast Coast fever when the parasites are either rare or absent; and instances are given of such reactions followed by recovery in experimental animals. Although the mild form of the disease seems associated at times with light tick infestation, it is proved that a few infected ticks also transmit a fatal East Coast fever. It is shown also that ticks fed on a bovine during a short or mild reaction can produce a virulent form of the disease in susceptible animals, and that mild reactors acquire an immunity to the virulent disease.

4. The experiments which are described indicate that ticks do not become infected with T. parva from the blood of a bovine for the first 4 days of the reaction period.

5. Evidence is produced which strongly suggests that T. parva tends to disappear from infected hungry ticks kept under laboratory conditions for about a year or more. The age of the tick would appear to be an important factor in the transmission of East Coast fever.

6. Attempts to break down the immunity to East Coast fever, and to produce “turning-sickness”, by massive infestations of ticks were unsuccessful; and further experiments on the transmission of East Coast fever by ticks which had fed on animals suffering from “turning-sickness” did not confirm earlier positive results.

These conflicting results may be explained by the observations that “turning-sickness”, associated with the blocking of the small capillaries of the brain by lymphocytes containing schizonts indistinguishable from the Koch's bodies of T. parva, may be divided into three stages, namely, acute, subacute and chronic; and it is suggested that infection of ticks with T. parva from “turning-sickness” could normally be successful only from animals in the acute and early part of the subacute stages since piroplasms disappear from the peripheral blood stream in the more advanced cases of “turning-sickness”.

7. The opinions of other workers on East Coast fever are discussed in the light of the experiments carried out at Kabete and of field experience in Kenya Colony. It is explained how the results can be applied to the movement of cattle from endemic areas over country free from East Coast fever.