Many reports indicate that anaesthesia affects several immunological functions that decrease the immune response, but the mechanisms involved are still unknown. We investigated the in vitro effect of halothane on human lymphocyte metabolism and plasma membrane function by evaluating the intracellular concentration of 3',5'-cyclic adenosine-monophosphate (cAMP), phosphodiesterase enzyme activity, NAD+/NADH intralymphocytic ratios and the degree of antibody and lectine-induced 'capping' of surface markers. Our results demonstrated an impaired lymphocyte capping of surface immunoglobulins and concanavalin A receptors 60 min after exposure to halothane at the concentration of 1% in oxygen. This phenomenon was reversible after 24 h and it was unrelated to the presence of adherent cells during the culture. Furthermore, halothane was able to induce a persistent increase in cAMP intracellular concentrations, which was reversible within 48 h. This effect was not dependent on adherent cells or on phosphodiesterase enzyme inhibition. Finally, no alteration in NAD+/NADH ratios after halothane exposure was observed.