The response of Saccharomyces cerevisiae to different concentrations of Pb2+ was investigated. The results demonstrated that the growth of S. cerevisiae in the presence of Pb2+ showed a lag phase much longer than that in the absence of Pb2+. The inhibition was dependent upon Pb2+ concentrations. The Pb2+ at a concentration of 5 mu M inhibited the microbial growth by approximately 30% with regard to control, whereas Pb2+ at concentration of 2 mu M did not have a significant effect on the microbial growth. The existence of Pb2+ did not perturb cell-protein synthesis and there was a good correlation between dry cell weights and total protein content (R-2=0.98). The RNA/DNA ratio in the microbial cells varied with Pb2+ concentration and there was a significant positive correlation between Pb2+ concentration and the RNA/DNA ratio. The microbial assimilation of ammonium ion was inhibited by the presence of Pb2+ in the medium; when Pb2+ concentration was 10 mu M, the microbial ammonium assimilation was inhibited about 50%, in comparison with the control experiment.