Lymphokine-activated killer (LAK) and natural killer (NK) cells were studied for their capacity to retain cytotoxicity after cryopreservation. LAK cells were generated by a 4-day culture of lymphocytes with recombinant interleukin-2 (rIL-2). Cytotoxicity was measured by 51Cr-release assay at effector:target ratios of 10:1 to 80:1. Cryopreserved LAK cells retained 58.8 to 87.4 percent of cytotoxicity, as compared with that in fresh control cells. Cryopreserved NK cell activity against K562 and Molt-4 targets was 45.7 to 67.9 percent of the respective values of the fresh control cells. The responsiveness of NK cells to polyinosinic-polycytidilic acid (poly I:C), interferon-alpha (IFN-alpha), or rIL-2 remained intact. Activated NK cell activity after poly I:C or IFN-alpha stimulation and that after rIL-2 were, respectively, comparable to and higher than the endogenous NK cell activity of the fresh cells. The composition of lymphocyte subsets as determined by flow cytometry using monoclonal antibodies did not change after cryopreservation, indicating that cellular loss of the given subsets did not occur during the procedure. The retention of substantial levels of cytotoxicity in cryopreserved LAK and NK cells may make them promising candidates as cytotoxic effector cells.