Two pyrethroids, permethrin and allethrin, are often combined for large-scale use in public health programs to control vector-borne diseases. In this study, the genotoxic potential of a commercial formulation of permethrin and allethrin was examined using cultured human peripheral blood lymphocytes (PBL). Genotoxicity was evaluated using the cytokinesis-block micronucleus cytome (CBMN cyt) assay by measuring the frequency of micronuclei (MN), nuclear division index (NDI), formation of nucleoplasmic bridges (NPB) and nuclear buds (NBUD), as well as apoptotic and necrotic cells. Human PBL were treated with different concentrations of a permethrin/allethrin mixture (1/0.01, 5/0.07, and 10/0.14 μg/ml) for 24 or 36 h. The highest concentration (10/0.14 μg/ml) of permethrin/allethrin mixture significantly increased MN frequency and percent apoptotic cells after incubations for 24 or 36 h. The NDI was markedly decreased in response to treatment with 5/0.07 or 10/0.14 μg/ml permethrin/allethrin for both 24 and 36 h. Exposure to the permethrin/allethrin mixture did not significantly alter formation of NBUD, NPB, or percent necrotic cells. The MN frequency was significantly correlated with the number of apoptotic and necrotic cells but inversely correlated with NDI. Data demonstrated that a mixture of permethrin and allethrin induced concentration- and time-dependent cytotoxic and genotoxic damage to human PBL in vitro.