Metabolic cooperation assays were conducted with Chinese-hamster-V79 cells at three separate laboratories using standardized protocols and chemicals of known tumor promoting activity. Each laboratory used identical lots of test chemicals, solvents, serum, medium, and trypsin as well as a standard protocol. The chemicals to be tested were either known tumor promoters or had related chemical structures. Among the test chemicals were phorbol esters, barbiturates, antioxidants, peroxides, bladder tumor promoters, alkanes, phenol and metabolites, and others. Agreement between laboratories was found for nine of 15 chemicals tested at three laboratories. For 23 chemicals tested in two laboratories, agreement was complete on 16 chemicals. Interlaboratory agreement appeared good when mean values for cloning efficiency for the entire study were examined, but the range of individual experiment means within a laboratory was quite large. The authors conclude that several characteristics of the assay may limit its usefulness for screening broad classes of chemicals for potential activity as tumor promoters. Many chemicals show organ specific effects and it may be unrealistic to expect any individual cell type to detect all such chemicals.