AbstractPregnant C3H/Anfcum mice were injected ip with 150 ug benzo-(a)pyrene (BP)/g body weight at the second trimester (12 days). Quantitative and differential changes were assayed in the peripheral blood leukocytes and erythrocytes at various times before and after mating and treatment. Within 5 days after injection, a 2- to 4-fold reduction in leukocytes was observed when compared to controls [corn oil (vehicle for BP)-injected pregnant females] which persisted into the 10th postpartum day. The erythrocytes were also significantly reduced but not to the same degree (1.2- to 1.5-fold). Depression in white blood cells is attributed to lymphocyte depletion since the granulocytes were virtually unchanged and the lymphocyte to granulocyte ratio, ordinarily>2 was 1 or