Insoluble metal phosphates were capable of inhibitory effects on plasma membrane H+-ATPase-mediated glucose-dependent H+ efflux from Saccharomyces cerevisiae. The relative toxicities of the different compounds used was Zn > Co which was the same order as that obtained for soluble chlorides of these metals, although the toxicity of the latter was manifest at lowe; concentrations. Inhibitory effects of the metal phosphates were dependent on the amount in suspension and increased with increasing preincubation time. The inhibitory effect of toxic metal compounds on glucose-dependent H+ efflux could be reduced or prevented by the addition of chlorides of calcium and magnesium, calcium being more effective than magnesium. The relative protective effects of Ca and Mg were similar for both soluble and insoluble compounds, and Ca could neutralize the influence of both cobalt and zinc phosphates. Although toxic effects could result from the solubilization of the insoluble metal phosphate compounds as a result of H+ efflux, releasing potentially toxic metal cations, there was no difference in the amount of metal cations released whether glucose was present or absent from the treatments. In addition, it was found that significant concentrations of soluble Co2+ and Zn2+ were released into metal phosphate suspensions in the absence or presence of cells with equilibrium concentrations in cell free suspensions (attained after 10-20 min) being about 60 mu M for Zn-3(PO4)(2) and about 65 mu M for Co-3(PO4)(2). In the presence of cells, a lower equilibrium concentration was attained in the presence of Zn, probably due to uptake by glucose-dependent and -independent processes. Such uptake was not observed for released Co2+ and could account for the higher toxicity of zinc phosphate compared to cobalt phosphate. Although some direct interactions with the insoluble phosphates must have been involved in overall inhibitory effects, it is concluded that a significant proportion of toxic effects resulted from free metal cations in solution, with protective effects of Ca and Mg resulting from competitive and stabilizing interactions at the cell surface.