Observed convergence of the Salmonella plate and pre-incubation assays when employing varying levels of S9.

Since the publication of the original Salmonella mutagenicity test protocol in 1975, a number of investigators have proposed that the addition of a pre-incubation step to the normal protocol increases the sensitivity of the assay. Data presented here for 4-dimethylaminoazobenzene and the more potent carcinogenic analogue 6-dimethylaminophenylazobenzothiazole show that simply varying the amount of S9 present in the plate assay can increase the sensitivity of the test as much as the addition of the pre-incubation step does.