It was found in bursectomized chickens that selective inhibition of IgM or IgG immunoglobulin synthesis occurs with equal frequency. Alterations in the immunoelectrophoretic pattern of immunoglobulins were described. Lymphocyte transformation induced by rabbit anti-IgM serum was inhibited in chickens lacking either IgM or IgG. Spleen lymphocytes from bursectomized chickens with a normal pattern of immunoglobulins exhibited transformation induced by anti-globulin serum like controls, despite a decreased humoral antibody response.