Intramuscular inoculation of 19-day old chickens with heterologous erythrocytes elicited significantly lower levels of agglutinins in embryonically infected birds when compared with control birds (reciprocal ha titer 10 vs. 100, respectively, P less than 0.01). The IgG component of this antibody was substantially less in experimental chickens, 25% vs. 70% in controls. Subsequent anamnestic antibody response in experimental birds was also significantly less than control chickens (reciprocal ha titers 25 vs. 512, respectively, P less than 0.02), with continued predominance of IgM antibody in experimental chickens (70% IgM-ha) vs. controls (60% IgG-ha). Induction of cell-mediated immunity by footpad inoculation of 46-day-old birds with diphtheria-tetanus toxoid in complete Freund's adjuvant failed to discern suppression of T cell function in previously infected chicks. Before hatch, the bursae of experimental and control embryos possessed a few IgM-producing lymphoid cells in equivalent numbers. There was also no difference in the relative population of IgM-producing cells in the medullary area of bursal follicles of 7-day-old hatchling chickens, approximately 70%. However, the mean density of IgG-producing cells in the bursae of experimental chickens was less than that of control chickens, 10% vs. 19%, respectively. Consistent with persistent embryonic viral infection and altered bursal function, only IgM-producing cells were detected in the spleens of 7-day-old experimental birds. These observations suggest that persistent embryonic viral infection disturbs the function of bursal-dependent immunopoiesis in the avian host.