Elimination of metabolic co-operation and the induction of sister chromatid exchanges are not properties common to all promoting or co-carcinogenic agents.

High densities of wild-type, 6-thioguanine sensitive V79 cells reduce the recovery of 6-thioguanine resistant (6TGR) cells when they are co-cultivated. This metabolic co-operation effect has been reported previously to be eliminated by the tumour promoter 12-O-tetradecanoylphorbol-13-acetate and the accompanying enhancement of 6TGR colony recovery has been postulated as a rapid assay for tumour promoters. Elimination of metabolic co-operation, however, does not appear to be a characteristic of either the promoting agents anthralin, iodoacetic acid, oleic acid and delta-haemolysin or the co-carcinogenic/carcinogenic agents griseofulvin, diethylstilboestrol and stilboestrol diproprionate. Moreover, with the exception of stilboestrol diproprionate, none of these agents induce sister chromatid exchanges, but are all, with the exception of delta-haemolysin, associated with limited numerical chromosome changes and in four cases low level aberration induction. None of the agents were mutagenic.