Cryopreserved and thawed lymphocytes can be used instead of fresh lymphocytes to avoid test-to-test variability in studies of fluctuations of natural killer (NK) and lymphokine-activated killer (LAK) activities as a function of time. We investigated the effects of 1-h versus 18-h resting of lymphocytes on their lytic activities, because the process of cryopreservation and thawing decreases NK and LAK activities. Lymphocytes from renal cell cancer patients receiving adoptive immunotherapy were studied. An 18-h versus 1-h resting period led to a significant increase in NK activity but had no significant effect on LAK activity. The presence of 1200 IU/ml interleukin 2 (IL-2) in the medium 1 h prior to and during the cytotoxicity (CTX) assay increased in vivo and in vitro IL-2-induced LAK activities. This phenomenon has been interpreted as IL-2 dependency of effector lymphocytes (J.A. Hank, P.C. Kohler, G. Weil-Hillman, N. Rosenthal, K. H. Moore, B. Storer, D. Minkoff, J. Bradshaw, R. Bechhofer, and P. M. Sondel. Cancer Res., 48: 1965-1971, 1988). We performed kinetic studies to assess the role of effector lymphocyte recruitment in these experiments. LAK activity was tested in the presence or absence of IL-2 during preincubations and CTX assays varying between 0 and 120 min. These kinetic studies showed that effector lymphocyte recruitment indeed contributed to the increased level of LAK activity when IL-2 was added to the CTX assay. A minimal incubation period of 30 min was required to detect recruitment of lymphocytes. Effector lymphocytes could be recruited for periods varying between 90 and greater than 240 min, depending on the lymphocyte donor. We conclude that: (a) in vitro, IL-2-mediated recruitment of lymphocytes due to presence of IL-2 in the CTX assay may lead to an overestimate of the actual LAK activity; and (b) in vivo, prolonged IL-2 infusion after the administration of activated lymphocytes seems warranted in order to recruit maximal levels of effector lymphocytes with LAK activity.