Spleen cells obtained from mice injected with cyclophosphamide (200 mg/kg body weight) suppressed the secondary IgG antibody response of memory cells to a T-dependent antigen, DNP-HGG, in Millipore diffusion chambers. Significant suppression (greater than 50%) was found from 5 to 14 days following cyclophosphamide treatment, with peak suppression (86%) on day 7. The primary IgM antibody response to DNP-Ficoll, a T-independent antigen, was not suppressed by these cells. In contrast, suppression was observed in the primary IgM response to sheep red blood cells, a T-dependent antigen. In addition, treatment of the suppressor cell population with anti-Thy-1 serum and complement did not inhibit suppressor activity. We concluded that the suppressor activity was not attributable to a typical T cell, and that the target of suppression was not a B cell. Preliminary evidence suggests that the suppressor activity is regulated, directly or indirectly, by a T cell.