Polyphenols with antimutagenic and anticarcinogenic properties are present in fruits, vegetables and legumes. In this study, the Salmonella typhimurium tester strains TA98 and TA100 were used in the microsuspension assay to examine the antimutagenic effect of phenolic compounds extracted from the common bean (Phaseolus vulgaris) against mutagenicity induced by aflatoxin B1 (AFB1). A dose-response curve was constructed for AFB1; from which a level of 40 ng AFB1/tube was selected for all antimutagenicity assays. The AFB1 and phenolic extract (PE) were not toxic to the bacteria at concentrations tested. In the case of PE, results were similar to the number of spontaneous revertants for TA98 and TA100. The inhibitory effect of PE against AFB1 mutagenicity was dose-dependent at the lower concentrations tested (2.5, 5, 10, 12.5, 15 and 25 microgram-equivalent (+)-catechin/tube for TA98; 0.5, 1, 1.5, 2.5, 5, 10 and 25 microgram-equivalent (+)-catechin/ tube for TA100). Further, a two-stage incubation procedure was used to investigate the potential interaction between PE and AFB1. The greatest inhibitory effect of the PE on AFB1 mutagenicity occurred when PE and AFB1 were incubated together. When the bacteria were first incubated with PE followed by a second incubation with AFB1, lower inhibition was observed. Lower inhibition was also observed when the bacteria were first incubated with AFB1 followed by a second incubation with PE. The results suggest that the mechanism of inhibition could involve the formation of a chemical complex between of PE and AFB1.