The effect of Pb on Saccharomyces cerevisiae cell structure and function was examined. Membrane integrity was assessed by the release of UV-absorbing compounds and by the intracellular K+ efflux. No leakage of UV260-absorbing compounds or loss of K+ were observed in Pb (until 1,000 mu mol/l) treated cells up to 30 min; these results suggest that plasma membrane seems not to be the immediate and primary target of Pb toxicity. The effect of Pb on yeast metabolism was examined using the fluorescent probe FUN-1 and compared with the ability to reproduce, evaluated by colony-forming units counting. The exposition of yeast cells, during 60 min to 1,000 mu mol/l Pb, induces a decrease in the ability to process FUN-1 although the cells retain its proliferation capacity. A more prolonged contact time (120 min) of yeast cells with Pb induces a marked (> 50%) loss of yeast cells metabolic activity and replication competence through a mechanism which most likely requires protein synthesis.