In a lymphocyte blast transformation assay, the response of feline lymphocytes to concanavalin A was suppressed 20-65% in the presence of inactivated feline leukemia virus. The decrease was not due to viral cytotoxicity, as determined by trypan blue viability counts, nor was the virus binding the concanavalin A and interfering with its mitogenic stimulation. The virus may be biochemically repressive in itself, interfering with cell-mediated immunity within the feline system.