Rejection of a xenograft by BALB/c mice results in a highly potent immune peritoneal cell population. By using two in vitro assays in parallel, it has been possible to show that there are at least two active cell types. The first assay detects the detachment of target cells from plastic, which is referred to as direct cytotoxicity. The second reflects the inhibition of incorporation of a deoxyribonucleic acid precursor, 125I-iododeoxyuridine, into target cells, which could occur either as a result of direct cytotoxicity or as a consequence of growth inhibition (cytostasis). With a rabbit antimouse brain serum which is cytotoxic for thymus-derived T cells, it has been shown that direct cytotoxicity requires T cells, whereas the cytostatic response can be largely independent of T cell function during the assay.