The effect of sera from 17 patients with Crohn's disease, 8 with ulcerative colitis or 5 with intestinal tuberculosis on the proliferative response of mouse spleen cells induced by phytohemagglutinin (PHA) was studied. Sera from patients with Crohn's disease markedly suppressed the blastogenesis of mouse spleen cells (S.I. = 6.8 +/- 2.0, % suppression = 83%), as compared with normal sera (S.I. = 41.0 +/- 5.2, p less than 0.001, % suppression = 0). Conversely, ulcerative colitis sera did not suppress the blastogenesis of mouse spleen cells (S.I. = 43.5 +/- 8.7, % suppression = -6%), nor the sera of intestinal tuberculosis (S.I. = 38.9 +/- 4.0, % suppression = 6%). Thus, we confirmed the possible existence of immunosuppressive factors in Crohn's disease. Moreover, immunosuppressive factors in Crohn's disease were characterized for biochemical properties. The approximate molecular weight is 45,000 estimated by diafiltration and gel filtration on a Sephadex G-75 column. Analytical isoelectric focusing showed an increased amount of acidic protein in fractionated sera (m.w. ranging 30,000-50,000) from patients with Crohn's disease and ulcerative colitis, in comparison with that in normal sera. Furthermore, the main peak of this acidic protein in Crohn's disease was an isoelectric point (pI) of 2.8, while the pI of that from ulcerative colitis was 3.0. These results suggest that qualitative differences of such acidic protein may serve to discriminate between the sera of Crohn's disease and ulcerative colitis.