The effect of Azathioprine (AZP), an immunosuppressive drug, on the function of blood monocytes and lymphocytes cultured in vitro, was studied. An attempt was made to correlate the effect of the drug on various functions of mononuclear phagocytes with the effect on the rosette forming ability of lymphocytes. The experiments indicate that AZP in concentrations below the level of cytotoxicity suppressed the rosette forming ability of lymphocytes as well as the digestive capacity of macrophages.