The effects of equimolal doses of cyclophospha-mide (CY), Isophosphamide (IP), 4-methylcyclophosphamlde (4-MCY), and phosphoramide mustard (PM) on murine hematopoietic spleen colonies and adoptively transferred antibody-forming cells in vivo were compared. Equimolal doses of the drugs produced significantly different effects. All the drugs exerted an increasing effect against the ability of adoptively transferred immunocompetent cells to produce a significant anti-sheep red blood cell titer as the length of time between cell transfer and drug administration was increased. The maximum effect was seen when a drug was given 48-72 hours after antigen and spleen cell transfer. CY and IP produced significantly greater immunosuppressive effects than did the other drugs at all times after cell transfer and at all doses administered. PM had the least immunosuppressive effect at each dose evaluated. Against hematopoietic spleen colonies, the cytotoxic effects of 4-MCY and PM were similar and, at most doses studied, significantly greater than the effect of either CY or IP. Inasmuch as PM is an active metabolite of CY, It appeared either that one of the prior metabolites of CY was responsible for this marked immunosuppressive effect or that due to differences in polarity, PM was differentially distributed within the two cell systems as compared to CY. The differences In hematopoietic effects among all drugs were much less than those seen against immunocompetent cells and were not dependent on time of drug administration.