b-Carboline alkaloids, found in medicinal plants, tobacco smoke and well-cooked foods, have shown a variety of actions in biological systems related to their interaction with DNA. Therefore, these alkaloids can be considered potentially mutagenic. In this work, the genotoxic, mutagenic, and cytotoxic activities of three aromatic b-carboline alkaloids Žharman, harmine, and harmol. and two dihydro-b-carboline alkaloids Žharmaline and harmalol. were evaluated by means of the Salmonellarmicrosome assay ŽSalmonella typhimurium TA98, TA97, TA100, and TA102. and SOS chromotest Ž Escherichia coli PQ37. with and without metabolic activation. Moreover, harman and harmine were analyzed by the micronucleus assay in vivo. It was shown that genotoxicity was inhibited by the addition of S9 mix for aromatic b-carbolines harman and harmol in TA97. However, harmine showed signs of mutagenicity only in the presence of S9 mix in TA98 and TA97 frameshift strains. In the SOS chromotest, only harman induced SOS functions in the absence of S9 mix. Dihydro-b-carbo- lines were not genotoxic in any of the microorganisms used. The negative responses obtained in the micronucleus assay indicated that harman and harmine were not able to induce chromosomal mutations. Keywords: b-Carboline; Genotoxicity; Micronucleus assay; Mutagenicity; Salmonellarmicrosome assay; SOS chromotest