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All organisms, from bacteria and yeast to humans, respond to physical and chemical stressors by increasing the synthesis of a small group of ’’cellular stress proteins.’’ We have developed a simple in vitro system for quickly screening environmentally relevant stressors to detect stress-induced proteins that are good candidates for biomarkers. Polyacrylamide gel electrophoresis was used to detect stressor-induced, concentration-dependent changes in cellular stress protein levels in two fish cell culture systems, whereas simultaneous in vitro neutral red uptake cytotoxicity assays measured the stressor’s effect on cellular physiology. There was a direct concentration-dependent relationship between sublethal cytotoxic effects and the increases in stress protein levels. Increases of 50 to 200% were detected in stress proteins from desert topminnow, Poeciliopsis lucida, hepatoma-derived cell cultures exposed to cadmium (six proteins) or copper (four proteins). Three proteins showed similar increases in winter flounder, Pleuronectes americanus, kidney cell cultures exposed to the same stressors. Increases in the evolutionarily conserved heat-shock protein hsp70 were detected in each experiment; its level increased with increasing stressor concentrations.