The in vitro effects exerted by tetroxoprim and sulfadiazine on T lymphocyte proliferation were evaluated at drug concentrations equal to and twice the plasmatic peak level of each drug. The two drugs did not exert any significant effect on spontaneous and mitogen (PHA) or monoclonal antibody (MAb anti-CD3) induced T lymphocyte proliferation at the studied concentrations, both when tested separately and combined. In addition the two compounds did not significantly interfere in gamma-interferon (IFN-gamma) production.