Changes in the frequency of sister chromatid exchanges (SCEs) in cultured mammalian cells have been considered as indices to the synthetic and sharp genotoxic and clastogenic potencies. Frequency of SCEs in cultured lymphocyte obtained from infants postnatal of around ten months was examined to evaluate the genotoxic or clastogenic potency of lactational exposures to some organochlorine pesticides such as HCHs, DDT and chlordane in this study. Frequency (median, min.-max.) of SCEs as the control culture treated with the solvent, DMSO, alone (SCE control) was 8.0/cell, 6.5-12.4/cell, that as the culture treated with 7,8-benzoflavone (SCEANF) was 11.8/cell, 9.1-15.6/cell and the difference between SCEANF and SCEcontrol, that is, delta SCEs was 3.9/cell, 0.2:7.2/cell. Lactational exposures (median, min.-max.) to the three organochlorine pesticides were as follows: HCHs; 341 mg/kg body weight, 43-1449 mg/kg body weight, DDT; 272 mg/kg body weight, 33-1361 mg/kg body weight and chlordane; 69 mg/kg body weight, 13-379 mg/kg body weight. The median exposure level to chlordane through the breast milk was one fourth to one fifth times lower than that to HCHs or DDT and probably due to such small amounts of it's intake, we could not find any correlation of chlordane with frequencies of SCEcontrol, SCEANF and delta SCEs in cultured lymphocytes of Japanese infants postnatal of about ten months. In the meantime, frequencies of SCEcontrol and SCEANF showed increasing and decreasing tendencies with the increasing exposure to DDT and also HCHs through the breast milk. In consequence, the frequency of delta SCEs was significantly lowered. Therefore, lactational exposure to DDT and HCHs seemed to elicit some genotoxic or clastogenic effects on Japanese general infants at ages of around the months. These compounds were also considered rather potent S-dependent clastogens than non-S-dependent ones. So, further detailed studies are needed not only to get conclusive findings but also from this viewpoint.