A single intraperitoneal injection of 4mg cyclophosphamide administered 2 days before the primary immunization of NMRI mice with 4 × 108 sheep erythrocytes led to a significant suppression of the primary immune response to this antigen, both at the cellular and humoral levels. Unlike this, the secondary response elicited 4 weeks after primary antigenic stimulation was found to be increased. To a lesser degree, the inhibition of the primary immune response was only demonstrable when the drug was given as early as 4 days before priming, although at that time the maximum in both the reduction of the spleen weights and the blood leukocyte counts were found. Again, the secondary response of the cyclophosphamide-treated animals was found to be increased. Killed cells of Bordetella pertussis, given simultaneously with the erythrocyte antigen, were capable to overcome partly the drug-induced inhibition of the primary immune response, and to increase the process of priming for the secondary immune reaction. Furthermore, the B. pertussis vaccine effected accelerated increase in both the extremely suppressed spleen weights and blood leukocyte counts.