Under carefully controlled conditions in a clinical research ward, conjugated primary bile-acid concentrations were measured by a 125I-radioimmunoassay every eight hours for four days in seven healthy control subjects and ten patients with hepatic disease (five with cirrhosis, three with primary biliary cirrhosis and two with sclerosing cholangitis). Two meals of approximately equal composition were consumed daily at 10 A.M. and 6 P.M. and blood was drawn at 4 A.M., 12 noon and 8 P.M.. With the exception of one patient, all subjects had greater postprandial than fasting serum bile-acid concentrations, with all healthy control subjects and most of those with hepatic disease showing evening values equal to or greater than the noon values. For the healthy control subjects, the mean values were 0.8, 1.4 and 1.9 μ, and for those with hepatic disease, 108, 140 and 133 μM. There were large fluctuations in serum bile acids (up to sevenfold) among patients with hepatic disease. These fluctuations were independently validated by finding corresponding changes in serum radioactivity derived from injection of a tracer (24-14C cholic acid) at the start of each study. To be consistent, especially for serial measurements, bile acids should be measured in blood taken at the same time of the day and at the same time relative to meals.