A brief account is given of the accepted view that riboflavin supplements in the diet of breeding stock lead to improved hatchability and that clubbed down is specific for a deficiency of this vitamin. Attention, however, is drawn to the apparent failure of various workers to achieve a hatching rate with a riboflavin supplemented diet substantially in excess of 80% of fertile eggs which appears to be the national average figure. An experiment is described in which range managed breeding stock received diets supplemented with riboflavin at varying levels, the lowest of which, in the light of available evidence, would be sufficient for good hatchability having regard to the riboflavin obtained by the stock from grass and their own droppings. The hatching rate of fertile eggs was maintained at about 80% with all rations and little change took place consequent on changing over from high to low or vice versa. Only a very small improvement was observed with a very high level of riboflavin. It is therefore concluded that either a figure around 80% hatchability represents the maximum influence of riboflavin on hatching rates or that there are generally other factor(s) present (or absent) in the embryo that prevent a higher rate being achieved. The evidence submitted suggests that this factor may be B12 or more probably, one of the unidentified factors in the A.P.F. complex, and that for hatching rates materially above 80% the A.P.F. complex must be increased above the levels presently supplied to range managed breeding stock, or the high hatching rates induced by high levels of riboflavin may greatly increase the birds' requirements for this factor(s) to a level well above that normally necessary.

Attention is also drawn to the incidence of clubbed down among dead-in-shell with varying levels of dietary riboflavin. It is concluded that the presence of this embryonic abnormality is rapidly and almost completely influenced by changes in the level of dietary riboflavin and that these changes take place much more rapidly than changes in hatchability, once a moderately high hatching rate has already been reached. It is also contended that high riboflavin levels will almost eliminate the incidence of clubbed down, although an already moderately high hatching rate will remain unaffected. For this reason it is argued that the presence of clubbed down in some dead-in-shell is no conclusive evidence that an improvement in hatchability will follow supplementation of the diet with riboflavin unless the hatching rate is below average.