The influence of purified hepatitis B virus surface antigen (HBsAg) preparations or of supernatants derived from PLC/PRF/5 cell line (which produces HBsAg) on human natural killer (NK) activity was examined. Lymphocytes pre-incubated with HBsAg and subsequently washed showed a significant decrease in NK cytotoxicity against K-562 target cells. This effect was reversible and dose-dependent. In addition, pre-incubation with either HBsAg or PLC/PRF/5 supernatants inhibited in a reversible manner lymphocyte--K-562 conjugates and the binding of B73.1 monoclonal antibody (MoAb), which recognizes Fc receptors on NK cells. This effect was not observed with HNK-1, T3, T4, T6, T8 and T11 MoAb. HBsAg was non-toxic to lymphocytes, and ineffective with K-562 target cells. Beta-interferon did not modify HBsAg-mediated inhibition, when added either before or during the contact with HBsAg. Moreover, no modification was observed when neutrophils (at various neutrophil:lymphocyte ratios) were added, even though HBsAg is known to stimulate neutrophils to produce oxygen radicals which may modulate NK activity. We speculate that HBsAg produces these effects by reacting into receptor sites (possibly Fc receptor sites) on NK cell membrane. The overall significance of our results in relation to hepatitis and hepatocellular carcinoma is discussed.