NK cytotoxicity of PBL from elderly persons increased in: (a) short-term assay, after previous IL-2 culture and (b) after prolonged contact (18 hours) with the target K-562 cells. Results are expressed as mean values, and each point is the average value of five to six samples. ○, short-term assay (2 hours); □, short-term assay (2 hours) with PBL cultured in IL-2 for 24 hours; •, long-term assay (18 hours).