Summary The objective of this study was to evaluate the effects of feline retrovirus protein on cat lymphocyte recognition of allogeneic cells in the mixed leukocyte reaction (MLR), using u.v.-inactivated feline leukaemia virus (FeLV) and a non-disease-producing endogenous cat virus (RD-114). Twenty micrograms of u.v.-inactivated virus protein from each virus were added to mixed leukocyte cultures. FeLV induced significant suppression of the MLR (P < 0.005) whereas no significant suppression was found using RD-114. Since feline leukaemia infection is associated with immunosuppression, the retrovirus proteins may be interfering with the response of immunocompetent cells when they are confronted with an altered host cell.