Four different in vitro cytotoxicity tests were compared: the MTT assay, the NR assay, the uridine incorporation assay and the measurement of total cellular protein. The comparison was done using the BHK-21 cell line and nine selected test chemicals (colchicine, amitriptyline, cycloheximide, 2,5-hexandione, mercury chloride, cadmium chloride, copper chloride, 2,4-dinitrophenol and chloroquine diphosphate). The concentration that induced 50% inhibition relative to the controls (IC(50)) was calculated for each test and chemical. The results from the cytotoxicity tests were generally in good agreement. However, for some chemicals the IC(50) values varied significantly between two assays. The largest variation was found for chloroquine diphosphate, where the IC(50) value for the NR assay was approximately eight times lower than the IC(50) value for the protein measurement.