NK-mediated cytotoxicity is regulated by a variety of cytokines and is thought to involve perforin and granzymes. The effects of IL-2 and IL-12 on the expression and activation of cytolysis were examined in freshly isolated human NK cells. A dose-dependent increase in cytolysis of the NK-sensitive target cell, K562, and the NK-insensitive but lymphokine-activated killer (LAK) cell-sensitive target, UCLA-SO-M14, was observed after short term culture of purified human NK cells in either IL-2 or IL-12. Moreover, the two cytokines often synergized to produce augmented lytic activity. A suboptimal dose of IL-2 (60 IU/ml) combined with IL-12 (2 U/ml) could induce lytic activity equal to twice the additive effect of each cytokine alone. Northern analyses revealed time-dependent increases in mRNAs encoding for perforin and granzymes A and B following treatment with IL-2 alone or IL-2 plus IL-12. IL-2 and IL-12 also synergized for the induction of granzyme mRNAs, in that treatment with both cytokines increased mRNA levels approximately 50% above the sum of each cytokine alone, as quantitated by phosphorimage analysis, and normalized to GAPDH gene expression. However, the synergy between IL-2 and IL-12 for the induction of mRNA was less dramatic than for lytic activity. Results of experiments in which cytokine-treated cells were pulsed with actinomycin D indicated that the increased granzyme and perforin gene mRNA levels in response to IL-2, IL-12, or the combination were not due to increased transcript stability. The data suggest that low doses of IL-2 and IL-12 synergize to augment NK- and induce LAK-mediated cytotoxicity and that this increase is associated with enhanced transcription of perforin and granzyme genes in a synergistic fashion.