Fig. 4

Results obtained under optimized conditions. The total protein content in the S9 Mix was set at 1.2 mg/plate (a common concentration for Ames tests), and the length of the pre-incubation period was 2 h for the main tests. The plots shown indicate the relative ratio of the average number of revertant colonies observed (treated: solvent control) in two plates and the number of spontaneous revertants. The number of spontaneous colonies per plate ranged from 96 to 128, varying slightly in the presence and absence of the S9 Mix in response to the different pre-incubation times used