Chickens injected intravenously (i.v.) with human adenovirus type 6 (Ad6) reveal a 2-17-fold increase in the number of plaque-forming cells producing antibody (Ab) against sheep red blood cells (SRBC) 2-6 days after virus infection. Further, polyclonal B-cell activation has been demonstrated by the quantitation of immunoglobulin-producing cells (IgPC) and cells producing immunoglobulin (Ig) of IgM isotype (IgPC mu) in the spleen of chicken inoculated with Ad6. Ad6 infection in chicken results in immunosuppression against SRBC when this unrelated antigen is given after virus infection. It seems that coincidence occurs between the B-cell mitogenic activation and the immunosuppression caused by Ad6, as the most pronounced change in both activities appears on the fourth day following virus infection. These findings suggest that the B-cell mitogenicity of the virus contributes to the impairment of the humoral immune response to SRBC.