In a previous study (CLARK and LAMONT 1976), 26 pregnant big brown bats were captured, caged, and fed uncontaminated mealworms until their litters were born. Immediately after parturition, female bats and litters were frozen. Five litters included at least one dead young, and these five litters contained significantly more of the PCB, Aroclor 1260, than did the 21 litters with only living young....The present study attempted to verify that Aroclor 1260 could cause stillbirths. I fed 18 of 36 pregnant big brown bats mealworms containing 6.36 ppm of Aroclor 1260 prior to birth of their litters. Both carcasses and litters of dosed females contained approximately 10 times more PCB than their respective controls, but no additional stillbirths resulted. Three of 18 control litters included dead young, whereas the comparable ratio among litters from dosed females was one of 18. Additional comparisons involving means of litter weight, adult female weight, parturition date, days in captivity, tooth wear, and percentage fat also failed to show any effect of the PCB....The association found earlier between PCB and dead young (CLARK and LAMONT 1976) was not one of cause and effect. In both studies, bats that had not been dosed showed greater PCB residues among younger females. Among control bats in the present series, females that produced dead young were significantly younger (that is, showed significantly less tooth wear) than other females. In sum, whereas dead young seemed to have been caused by greater residues, these two factors were actually independent of each other but associated with a third factor--age of the female parent bat.