Previous studies by us have shown that preventing development of the bursa of Fabricius by dipping fertile eggs in testosterone propionate (TP) on the third day of incubation results in elevated levels of IgM in plasma beginning 2 to 4 weeks after hatching. In the present study we have found that such chicks repeatedly immunized with sheep red blood cells (SRBC) produce increasing amounts of IgM antibody but no IgG antibody, in contrast to normal birds which show increasing antibody responses only in the IgG class. TP birds raised in a pathogen-free (PF) environment from hatching until 10 weeks of age have significantly lower levels of IgM than TP birds raised in a conventional environment. The IgM levels approach those of the latter birds at 15 weeks, 5 weeks after being returned to a conventional environment. Further experiments with infusion of normal and immune IgM and IgG into normal birds demonstrated some enhancement of IgM levels by IgM but marked depression of levels of immunoglobulin and antibody in both classes by IgG. The results strongly support the hypothesis that elevated levels of IgM in bursaless, TP-treated birds result from immune responses to environmental antigens unregulated by feedback from IgG. They provide further evidence that the bursa is essential for the normal switch from IgM to IgG production but not for the development of lymphoid cells producing IgM.