Abstract 4-Hydroperoxycyclophosphamide (4-OOH-CYP) is spontaneously converted in aqueous solution to 4-hydroxycyclophosphamide (4-OH-CYP), the major active metabolic of cyclophosphamide. We studied the dose related effects of in vitro treatment with 4-OOH-CYP on human T- and B cell-mediated immune responses. T-cell proliferation to mitogens and alloantigens was only partially inhibited even relatively high-doses of 4-OOH-CYP(> 6 – 12 μg/ml). In contrast cytotoxic functions of activated T-cells and natural killer (NK) cells were inhibited at lower doses (3–6 μg/ml). PWM induced in vitro synthesis of IgG by B-cells was inhibited at <3 μg/ml of 4-OOH-CYP. These data indicate that at 4-HOO-CYP has selective, dose-dependent effects on human T and B cells in vitro.