Azathioprine (AZ) and Methotrexate (MTX) inhibit thymidine incorporation in mixed lymphocyte reaction (MLR) at non-cytotoxic concentrations. AZ is active only when introduced in the culture during the first 24 hr, whereas MTX is still active when introduced at day 4. These data indicate that AZ might act on antigen recognition by T cells, as previously suggested by its effects on inhibition of rosette formation, and not on cell proliferation, as MTX probably does. Stimulation of thymidine incorporation has been observed at low drug concentrations, especially when the drugs were added more than 24 hr after the beginning of the culture, and most particularly for MTX added 6 days after start of the culture.