We described previously a simple test on petri plates for detecting chemical carcinogens as mutagens, using an especially sensitive set of bacterial strains to detect mutagenic acitivty and a mammalian liver extract for carcinogen activity. We now extend the utility of the method by introducing two new bacterial strains which can detect with great sensitivity many carcinogens which we did not detect before or detected with less sensitivity. Among these carcinogens are aflatoxin B-1, sterigmatocystin, benzyl chloride, benzo[a]-pyrene, 7,12-dimethylbenzanthracene, 1'-acetoxysafrole, and the nitrofuran food additive furylfuramide (AF-2). The new strains TA100 and TA98 contain an R factor plasmid, pKM101, in our standard tester strains TA1535 and TA1538. The R factor increases mutagenesis with certain mutagens, but not others. We present evidence that the mutagens that become more effective work through an error-prone recombinational repair.