Abstract 14C- Chlorella protein was used as a marker to avoid the uncertainties of the conventional N balance study of protein digestibility. In the mouse infected with Nematospiroides dubius and the rat with Nippostrongylus brasiliensis, protein digestion was nearly complete when the hosts were killed about 20 hours after feeding the test meal. In the sheep infected with Trichostrongylus colubriformis, the function of the small intestine was examined by two methods after feeding into the abomasum and collecting from the distal ileum. Mice and rats were reared on either 21% or 9% protein. In none of these infections was there any depression of protein digestion. Nor was there any evidence that the proportions of radioactivity in the distal ileum of the rat arising from proteins, peptides, and amino acids were changed by the infection. The results are discussed in relation to the poor utilization of food in nematode infections reported by others. There was a small but statistically significant fall in the mean mucosal dipeptidase activities in infected sheep compared with the greater depression of activity reported previously, for other enzymes in rats with nippostrongylosis. The difference in sheep was due to lower activities in the proximal small intestine, the site of infection. The extent and degree of villous atrophy was greater in the sheep with trichostrongylosis than in rats with nippostrongylosis.