AbstractBased on the findings of Nagao et al [1978] that phenacetin is negative in the standard Ames test with Aroclor induced rat S‐9 and positive with hamster S‐9, the test was performed with a mixture of rat/hamster S‐9. Phenacetin was mutagenic with the mixture. The activity of the mixture was compared to the rat S‐9 alone with low concentrations of 2‐aminoanthracene (a strong promutagen for Salmonella typhimurium TA 1535, TA 100, TA 1537, and TA 98), nitrosopiperidine (a weak promutagen), and 1, 2 epoxybutane (a weak, directacting mutagen). Except for an increased mutagenic activation by the mixture with nitrosopiperidine the mixture was comparable to the rat S‐9 alone, indicating that replacing rat S‐9 with a rat/hamster S‐9 mixture in the standard Ames test could increase the sensitivity of the test without interfering with r