Generation of natural killer (NK) activity in response to a partially purified preparation of rat interleukin-2 (IL-2) was compared in spleen cells derived from young (8-10 weeks old) and old (greater than 2 years old) female C57BL/6 mice. Significant NK activation was observed in both young and old mouse spleen cells incubated with 100 U IL-2/ml for 1-4 days, but the levels of cytotoxic activity generated in old mouse spleen cells was always lower than those of similarly treated young mouse spleen cells. Differences in IL-2-induced NK activation in old and young mouse spleen cells was obtained irrespective of the concentration of IL-2 used (25-400 U/ml). Quantitative comparisons indicated that old spleen cells activated by 3 day incubation with IL-2 acquired about two-fold higher NK activity than fresh young mouse spleen cells but still had only one-fourth of the levels of NK activity attained by IL-2-activated young mouse spleen cells. Cytotoxic activity of IL-2-activated young or old mouse spleen cells were totally abrogated by anti-asialo GM-1 antiserum + C but not by anti-Ly-2 + C treatment, indicating that the activated cytotoxic cells fell in the NK cell category. An analysis of NK precursor (NK-p) frequency by limiting dilution assay indicated that the NK-p frequency was about 4-fold higher in young as compared to old mouse spleen cells. The level of cytotoxic activity attained per NK-p cell was not significantly different for NK-p cells of old or young mice.