The frequency of peripheral blood cells expressing the perforin gene or the granzyme B gene was evaluated by in situ hybridization in nine patients suffering from metastatic melanoma and treated with recombinant interleukin-2 (rIL-2). A spontaneous expression of both genes was detected in five to seven patients. rIL-2 administration increased the frequency of positive cells in all patients (P < 0.03 for each gene), the highest frequency being reached in the patients who already expressed these genes prior to rIL-2 treatment (P < 0.02). Expressions of the granzyme B gene and of the perforin gene were strongly correlated before IL-2 treatment and they were similarly affected by rIL-2 administration. In contrast, their modification under treatment did not correlate with that of CD56+ cell counts, of natural killer activity and of sCD8 release. This indicates that perforin and granzyme B gene expressions are markers of cytotoxic cell activation independent of those previously described, and that they should be further evaluated in patients with malignancies to delineate their potential value in predicting clinical outcome.