Protein supplementation of silage diets using fishmeal can increase both the voluntary intake of silage and liveweight gain in young growing calves. In association with these responses, increases in ration digestibility have also been observed which are not predicted by ARC (1980) (Bax, J.A. and Offer, N.W., 1982). Similar unpredicted responses have also been observed in older cattle (Waterhouse, A., Laird, R., Holliday, R.J., 1983). ARC (1980) assumes that increases in digestibility following supplementation with RDP occur only if the basal diet provides less than 7.8 g RDP/MJ ME irrespective of the source of RDP. The objective of the present study was to subject the microbial requirements proposed in ARC (1980) to a critical evaluation.