Natural killer (NK) cell activity against K562 cell line, and interleukin-2 (IL-2) activity in supernatants from lectin-activated PBMC cultures from 17 patients with acute hepatitis B in the early phase of illness were studied. These patients showed enhanced NK cytotoxicity and higher levels of IL2 activity as compared with control subjects. There was a positive correlation between cytotoxicity values and levels of IL2 activity. Furthermore, in the recovery phase of illness there was a tendency towards normalization in both parameters. When patients were divided in accordance with markers of HBV replication, HBV-DNA positive patients showed increased NK cell activity and IL2 levels as compared with the control group, whereas in HBV-DNA-negative patients no differences were found. However, no differences were found between patients with HBeAg and patients with anti-HBe. These results suggest that natural cytotoxicity is increased early in the course of acute hepatitis B, while NK cell activity returns to normal later, during convalescence. Enhanced NK cell activity appears to be secondary, at least in part, to increased production of IL2. Natural cytotoxicity may be one mechanism that controls the HBV infection before other cytotoxic mechanisms become fully operative.