Conditioned immunosuppression was induced in C57BL/6 mice by pairing saccharin drinking (conditioned stimulus) with an i.p. injection of 250 mg/kg cyclophosphamide (unconditioned stimulus). Conditioned mice showed depressed mitogen-induced lymphoproliferation. This depression was significant for T-cell--but not for B-cell--proliferation. Mice also exhibited depressed antibody production against sheep red blood cells until 15 days after immunization. The kinetics of serum antibody levels suggest that conditioning amplifies the immunosuppressive effects of cyclophosphamide. Furthermore, we show in this report that a depression of cellular and humoral components of the immune system can be observed when conditioning is performed after immunization, the latter being not paired with the conditioned stimulus.