Considerable evidence documents the importance of co-factors, including the immune response, in expression of oncogenicity of tumour viruses. To determine whether a common protozoal infection that can depress lymphocyte function alters manifestations of oncogenic virus infection, a mouse model of Toxoplasma infection with depressed T lymphocyte function was developed. In this model, Toxoplasma depressed blastogenic transformation to the T-cell mitogen Concanavalin A and primary antibody response to sheep red blood cells which requires T cell help. Uninfected and Toxoplasma-infected mice were then infected with Moloney leukaemia or Moloney sarcoma viruses and development of lymphoma and sarcoma were evaluated. Toxoplasma infection, which induced depression of T-cell function, decreased the incidence of Moloney sarcoma virus induced rhabdomyosarcomas but did not alter progression or regression of tumour in those mice that developed tumour. Conjoint infection with Toxoplasma and Moloney leukaemia virus did not increase incidence of lymphoma when compared with incidence of lymphoma in mice infected with Moloney leukaemia virus alone.