Immunoregulatory lymphokines IL-2 and IL-4 produced by IL-2- and IL-4-cDNA-transformed somatic cells were used for LAK cell generation. The cytolytic effect of LAK cells activated for 3 days with IL-2 and measured by Cr-51 release of three types of target cells, YAC-1 (NK-sensitive), C1498 (NK-resistant) and X63-Ag8.653 (plasmocytoma cell line utilized for insertion of the IL-2 and Il-4 genes), was significantly depressed by adding IL-4 during the activation period.