Different methods of measuring cytotoxicity have been investigated in order to establish a test system for assessment of the toxicity of water samples. Ideally this should be highly sensitive and rapid to perform. Four variations of the neutral red uptake test have been compared with the MTT test and ATP determination. Chinese hamster V79/4 cells were used as the test system with 10 model toxins. All tests gave essentially similar results, with linear regression analysis producing correlation coefficients in excess of .93. No single test was most sensitive to all 10 compounds. The neutral red uptake and MTT tests could be performed in a single working day (approximately 8 h) if test compound and cells were plated out simultaneously. These tests are preferable to ATP determination, which is a complex and lengthy procedure, requiring expensive reagents.