The effect of bovine milk casein digests on the ability of thioglycollate‐elicited mouse peritoneal macrophages to produce nitrite and to ingest latex beads was investigated. Intactαs1‐casein and κ‐casein significantly inhibited nitrite production by the macrophages. The inhibitory activity of the latter was little influenced by trypsin or chymotrypsin digestion, while that of the former decreased with increasing proteinase digestion time. In contrast, pepsin digestion changed the inhibitory activity of a s1‐casein and κ‐casein into an enhancing activity. Intact ß‐casein, however, had an enhancing effect on nitrite production, and its activity increased noticeably when the protein was digested with pepsin. The enhancing activity of pepsin digests of each casein component reduced when the digest was treated with carboxypeptidase A. Moreover, a trypsin or chymotrypsin digest of α s1‐casein and κ‐casein inhibited ingestion of latex beads by the macrophages, while a pepsin digest of each casein component enhanced it. These results suggest that the neonate ingestion of infant formula made with bovine milk might reduce resistance to infection via suppression of macrophage functions, such as antimicrobial nitrogen intermediate production and phagocytosis in their gut mucosa, since it is known that gastric secretion and pepsin activity are weak in neonates and the major portion of the caseins ingested by neonates leaves the stomach after minimal digestion.