The effect of in utero TBTO exposure on blood composition in neonates, pups during weaning, and dams in the same period were investigated in mice. The dams were gavaged at dose levels 0, 5, 10, and 20 mg TBTO/kg body weight on gestational days 6–15. At birth, litters were culled to eight pups, and blood analysis was performed on excess pups. On d 7, 14, and 21 the entire litters were sacrificed and blood (WBC, RBC, HGB, HCT, MCV, MCH, MCHC, and differential leukocyte count) of dams and pups was analyzed. Blood analysis of neonates revealed enhanced WBC count and altered red‐cell parameters. Increase in WBC persisted until postnatal day (pnd) 21, while red‐cell parameters regained normal values on pnd 7. No significant differences were found in thymus and spleen weights in the treated groups. Dams presented no alteration in blood composition or of spleen or thymus weights. The results of this investigation revealed that in utero exposure to TBTO induces a nonspecific alteration of hematological parameters in mice, but further studies are necessary to understand whether our results are either dependent on maternal status or a direct effect of TBTO on embryonic tissue.