The influence of azathioprine and cytosine arabinoside on humoral and cell-mediated immunity was investigated. In these studies, two tissue culture systems which allow the generation either of a thymus-independent humoral immune response against DNP or of a thymus-dependent cell-mediated cytotoxic response against alloantigen were used. Whereas azathioprine inhibited the in vitro induction of cytotoxic lymphocytes in concentrations 300-fold lower (50% inhibition concentration: 0· 3 μg/ml) than that necessary to block the humoral anti-DNP response, cytosine arabinoside had no such selective effect. For cytosine arabinoside the 50% inhibitory concentrations were 0· 35 μg/ml and 0· 25 μg/ml for the thymus-independent humoral immune response and the thymus-dependent cell-mediated immune response respectively. Neither drug had any effect on the effector phase of either antibody-forming cells or of cytotoxic lymphocytes.