Cell-mediated immunity in acute nonlymphocytic leukemia, either at initial presentation or in subsequent relapse, was determined among 64 patients by recall-antigen skin testing and in 37 patients by attempted sensitization with dinitrochlorobenzene (DNCB). Thirty (46.9%) of the 64 skin-test batteries displayed reactivity to at least one established antigen, and 17 (51.5%) of the 33 evaluable patients were successfully sensitized with DNCB. Twenty-six (78.8%) of the 33 patients tested with both DNCB and recall antigens reacted to either one or both. The leukemic patient’s subsequent prognosis, in terms of attainment of complete remission of his or her leukemia, duration of remission, and survival could not be correlated with the pretherapy demonstration of intact recall-antigen or DNCB cell-mediated immunity. Serial, rather than single, determinations of the state of delayed hypersensitivity may be more meaningful in terms of prognosis in patients with acute nonlymphocytic leukemia. However, increasing chemotherapeutic efficacy may obscure whatever contribution, if any, intact cell-mediated immunity makes to the eradication of malignancy.