Michigan dairy farm residents ate farm products containing polybrominated biphenyls (PBB's) after the accidential contamination of animal feed with the chemical in that state in 1973. The circulating blood lymphocytes of these residents show significant changes. Abnormalities include decreases in the numbers and percentages of peripheral blood lymphocytes that form rosettes with either sheep erythrocytes alone or with sheep erythrocytes sensitized with antibody and complement, increases in lymphocytes with no detectable surface markers ("null" cells), and altered responses to tests designed to evaluate functional integrity of the cells. There appears to be no consistent correlation between the concentration of PBB's in the plasma and the altered lymphocytes. Studies showed that in Wisconsin dairy farm residents and healthy individuals in the New York area who were not exposed to PBB's there were no such abnormalities.