The plasma protein binding of ketoconazole, an oral antifungal agent of a weak basic nature, was measured after the addition of the drug (10 micrograms.ml-1) to serum from 35 healthy individuals, ten patients with chronic renal disease and seven patients with hepatic cirrhosis. The percentage of free ketoconazole was markedly increased in patients with chronic renal disease and in patients with hepatic cirrhosis, when it was compared with the group of healthy volunteers (7.33 +/- 0.11 in renal patients; 6.12 +/- 1.43 in hepatic patients compared with 2.93 +/- 0.12 in healthy individuals). The binding ratio of ketoconazole in health and disease was significantly related to plasma albumin concentration, but not to plasma alpha 1-acid glycoprotein (AAG) concentration. Moreover, ketoconazole binds to isolated human serum albumin in a greater proportion but does not bind to isolated AAG indicating that human serum albumin is the major binding protein for this drug in plasma.