PBB found in relatively low levels among animals present on a cross-section of Michigan farms during the time PBB was inadvertantly added to dairy feeds had no effect upon these animals' milk production, body weight, weight gain, breeding and reproductive performance, incidence of commonly experienced health problems, calving rate, and the health of their calves. No significant differences in these vital areas could be seen between Michigan animals exposed to PBB and equivalent Wisconsin animals which had not been exposed to PBB when both groups were subjected to equivalent management practices. No pattern of gross or histopathological lesions was seen upon necropsy between test animals and control animals.