Daunomycin, N-ethyl-N`-nitro-N-nitrosoguanidine, 9-aminoacridine and cumene hydroperoxide served as strain specific positive control compounds in the absence of S9 mix. 2-Aminoanthracene and benzo[a]pyrene were used for testing the bacteria and the activity of the S9 mix. Each treatment with the substances used as positive controls led to a clear increase in revertant colonies, thus, showing the expected reversion properties of all strains and good metabolic activity of the S9 mix used.