The in vitro immunologic parameters of both bone marrow-dependent (B) and thymus-dependent (T) cells of patients with no demonstrable immunologic abnormality and to whom only azathioprine (az) was given for a year in an uninterrupted course, were evaluated. No significant or consistent changes were detected in T cell population as tested by the in vitro proliferative responses of leucocytes to phytohaemagglutinin (PHA) and to allogeneic leucocytes in the mixed leucocyte reaction (MLR). No significant changes were observed in the levels of serum immunoglobulin (Ig) G, A or M. Significant suppression of B cells was noted: suppressed in vitro proliferative responses to heterologous anti-Ig antisera and to pokeweed mitogen (PWM). The suppressed capacity of B cells to proliferate was not due to a plasma factor but probably to a qualitative defect; normal binding capacity of B cells to 125 I-anti-Ig but their inability to proliferate in vitro upon interaction with the anti-Ig. In evaluating the efficiency of az therapy attention should be directed to the evaluation of the in vitro immunocompetence of B cells rather than that of T cells.