Previous investigations have shown that Pb exerts immunotoxic effects. Object of this study were Th1 and Th2-type immune responses of mice to Pb exposure. Adult Swiss male mice were administered 0, 40 and 400 mg/l of Pb (as acetate) in drinking water for 14 days. At the end of the treatment, blood Pb was determined and two Th1 cytokines (IL-2, INF-gamma) and one Th2 serum cytokine (IL-4) were measured. A significant increase in IL-4 production was observed in the mice exposed to 40 mg/l of Pb, while a further increase in IL-4 production was associated with a decrease in INF-gamma production in mice exposed to 400 mg/l of Pb. On the other hand, Pb exposure did not induce changes of serum IL-2 (involved also in the Th0 immune pattern). Our findings indicate that low level Pb exposure enhances a Th2 response. A high Pb does can either stimulate the Th2 immune activity or reduce the Th1 response; the result is an imbalance between Th1 and Th2 activation.