Investigations to determine the inhibitory activity on the Ca(2+)-transport-ATPase of human erythrocyte membranes were performed with various compounds of toxicological significance, mostly chlorinated and mainly used as biocides, such as phenol, 4-chlorophenol, 2,4-dichlorophenol, 2,6-dichlorophenol, 3,4-dichlorophenol, 2,3,4-trichlorophenol, 2,3,4,5-tetrachlorophenol, pentachlorophenol (PCP), captan, folpet, captafol, (+)-camphene, toxaphene, dichlorodiphenyltrichloroethane (DDT), lindane, endrin, dieldrin, alpha-endosulfan, beta-endosulfan, paraquat, diallate, 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). Some of the compounds investigated display an inhibitory effect on the Ca(2+)-transport-ATPase at very low concentrations. The in vitro results obtained in this enzyme assay can be correlated directly with the results of other in vitro assays and with the results of in vivo investigations in different species in which an inhibitory effect on various biological functions is observed. Therefore, an inhibitory effect on the Ca(2+)-transport-ATPase indicates a toxic effect of these compounds to cell functions. Since the inhibitory effect of these compounds can be measured rapidly and the enzyme is easy to handle, it might be a useful tool to screen the toxic effects of various compounds on cell function. The aim of the authors was to investigate the usefulness of this screening test system for the characterization of the cellular toxicity of various compounds.