First generations of milk protein exempted mice (6–8 weeks old) were fed with casein at 100 mg per day. At 0, 2, 4, 6, 8, 12 and 20 weeks of feeding, a group of eight mice was sacrificed to recover spleen lymphocytes and intraperitoneal mastocytes. In vitro lymphocyte3H‐thymidine incorporation in the presence of intact casein, peptic or peptic‐tryptic‐chymotryptic hydrolysate was significantly higher at eight and 12 weeks of feeding (p<0.05). The number of the sensitized mice increased from 39% in the control group to 61% in the experimental groups. The three antigens possessed a statistically equal capacity to stimulate lymphocyte proliferation. Measurements of plasma and culture supernatant IgG antibody production exhibited an elevated enzyme‐linked immunosorbent assay value starting from the eight week of casein feeding. Casein feeding also provoked histamine accumulation in the mastocytes which occured significantly after four weeks of feeding (p<0.001). The fact that the lymphocytes recognized not only intact casein but also its digestive enzyme hydrolyzates indicate that intestinal digestion and absorption processes could neither totally block the antigen passage nor destroy their epitopes.