Frequency of sister chromatid exchanges (SCEs), which has been considered an index to the synthetic and sharp genotoxic and clastogenic potencies, was examined by using the infant lymphocytes postnatal of around ten months, in order to evaluate the genotoxic or clastogenic potency of exposures to PCBs and dioxins through the breast milk. SCE frequency (mean +/- S.D.) as the control culture treated with the solvent, DMSO, alone (SCEcontrol) was 8.3 +/- 1.1/cell and that as the culture treated with 7,8-benzoflavone (SCEANF) was 11.9 +/- 1.5/cell. In addition, the difference between SCEANF and SCEcontrol, that is, delta SCEs was calculated 3.6 +/- 1.5/cell. The intake of PCBs (mean +/- S.D.) through the breast milk in these infants was 133 +/- 85 mg/kg body weight and that of dioxins 24 +/- 13 TEQ-ng/kg body weight. PCBs intake in the average was about 5,500 times greater than that of dioxins. We however, could not find any significant effect of PCBs on the frequencies of SCEs. Dioxins, on the contrary, showed a significant negative relationship to the frequency of delta SCEs. Therefore, exposure to dioxins through the breast milk seemed to elicit some genotoxic or clastogenic effects on Japanese general infants postnatal of around ten months.