The proliferative phase of the primary response of mice to sheep red blood cells (SRBC) was used to study quantitatively the schedule-dependent effect of four phase-specific cytotoxic agents: 1-μ-d-arabinofuranosylcytosine (ARA-C), methotrexate (MTX), 5-fluorodeoxyuridine (5-FUDR), and 5-fluorouracil (5-FU). The multiple injections of ARA-C given 24-60 hours after SRBC injection caused a 100-fold greater inhibition in the production of direct hemolytic plaque-forming cells (PFC) on day 4 than the maximum inhibition obtained by a single injection. A similar, but lesser, enhancing effect was observed with 5-FUDR. Although a single injection of either MTX or 5-FU given 24 or 48 hours after the immunization alone was effective in suppressing PFC production, multiple injections of MTX at 6-hour intervals could cause the same degree of inhibition with a smaller total dose. In contrast, there was no difference for 5-FU, whether given as a single injection or in a divided dose. When only two doses of cytotoxic agents were used, 8- and 14-hour intervals with ARA-C and MTX, respectively, gave the maximum degree of inhibition.