University of California (UCD) line 140 chickens, previously described to have an inherited 7S Ig deficiency and dysgammaglobulinemia associated with early bursal defects, and autoimmune phenomena were examined for the presence of suppressor cells. Pokeweed mitogen (PWM) stimulated UCD 140 peripheral blood lymphocytes (PBL) to synthesize IgM but not 7S Ig. In co-culture, UCD 140 PBL specifically inhibited 7S Ig but not IgM synthesis of PWM-stimulated PBL from normal birds. Normal 7S Ig synthesis usually was suppressed 40 to 80%, although PBL from some normal birds were not suppressible. Adoptive transfer of UCD 140 spleen cells to normal chicks resulted in early and transient decreased serum 7S Ig levels. Homology at the B locus (major histocompatibility complex) was not required for suppression. Higher levels of suppressor cells were found in UCD 140 than in normal birds as early as 13 days of age. The suppressor cells were found to be T cells as indicated by inactivation with anti-T antiserum but not with anti-B antiserum, and by suppression associated with T cell fractions obtained by nylon wool fractionation. UCD 140 B cells, obtained by nylon wool fractionation, were stimulated with PWM to synthesize 7S Ig when provided with normal T cells. These findings are discussed with particular reference to early bursal anomalies.