A group of cytotoxicity tests that detect alterations in cell metabolism were applied to obtain a preliminary classification of test chemicals, based on their main mechanism of toxicity. V79 cells were exposed to toxic compounds in 'acute' treatments (up to 2 hr) and the specificity of different endpoints of cytotoxicity was compared. We tested five directly acting chemicals: butylated hydroxyanisole, butylated hydroxytoluene, cycloheximide, potassium dichromate [Cr(VI)] and dinitrophenol using four assays measuring; [(3)H]thymidine uptake and incorporation into DNA, [(3)H]leucine incorporation into proteins, ATP pool size and cellular energy charge, and oxygen consumption. In the thymidine-uptake test the radioactivity of the nucleotide pool was hardly affected by low concentrations of the toxic chemicals and was reduced by about 20-30% at the 10(-4)m concentration, except by butylated hydroxytoluene; the latter which caused a marked inhibition of [(3)H]thymidine uptake. Thymidine incorporation into DNA was more specifically altered, showing a net inhibition by potassium dichromate, in a concentration-dependent fashion, and by butylated hydroxytoluene. Protein synthesis was more inhibited by potassium dichromate and butylated hydroxytoluene than by the specific inhibitor, cycloheximide. All chemicals reduced ATP concentration, but the energy charge was scarcely affected, reflecting a parallel decrease of the other adenine nucleotides. The assay measuring early changes in oxygen consumption produced the expected results with dinitrophenol, the antioxidants and potassium dichromate, and showed that cycloheximide also inhibits mitochondrial respiration, in agreement with the observed fall of intracellular ATP. All the chemicals tested induced a range of effects on the metabolic parameters analysed but specific pathways of toxicity may be inferred by comparing the results of the individual tests.