Serum activity of SGOT, SGPT, LDH, and alkaline phosphatase was measured in 614 Michigan adults exposed to PBB and 141 Wisconsin adults not so exposed. The Michigan group had higher prevalence of abnormal SGOT (p less than 0.005) and SGPT (p less than 0.005). A clear sex difference was observed. Michigan men had a higher prevalence of abnormal SGPT (p less than 0.005) and LDH (p less than 0.005) than Michigan women, and a higher prevalence than Wisconsin men of abnormal SGOT (p less than 0.005) and SGPT (p less than 0.01). These differences could not be ascribed to differing patterns of alcohol consumption, laboratory error, or choice of criteria for normality/abnormality. Seven Michigan subgroups were defined on the basis of the criteria by which they had been selected to participate. The two subgroups who were essentially self-invited did not differ from the remaining five randomly selected subgroups combined in prevalence of these abnormal liver function tests. Based on 364 serum PBB analyses thus far analyzed of the 614 Michigan participants, no obvious relationship between serum PBB values and liver function tests was observed. However, this is a tentative conclusion that will be further evaluated when remaining serum PBB analyses are completed. The greater prevalence of abnormal SGPT and SGOT among Michigan dairy farm residents compared to the Wisconsin dairy farm residents is tentatively ascribed to the former group's exposure to PBB.