1 The free fraction of azapropazone in the plasma of 37 healthy volunteers ranged from 0.0027 to 0.0070 (0.0044 + 0.0009, mean s.d.). The principal binding protein was found to be albumin. 2 In 27 patients with various degrees of renal failure the free fraction values of azapropazone were markedly enhanced (0.0260 + 0.0239, mean + s.d.) and increased more than tenfold in some patients. There was a weak correlation (r = 0.46, P<0.05) between the free fraction and the clearance of endogenous creatinine. Such correlation was not found for serum creatinine, serum albumin, serum uric acid and serum urea nitrogen. 3 In 32 patients with chronic liver disease the free fraction values of azapropazone were also markedly higher (0.0210 + 0.0242, mean + s.d.) than in healthy subjects. There were statistical significant correlations between the free fraction values and the prothrombin complex activity in the plasma (r = 0.40, P<0.05) and the total bilirubin concentration in the plasma (r = 0.90, P<0.001), respectively. Such correlation was not found for serum albumin, serum glutamic oxalacetic transaminase, serum -y-glutamyl transpeptidase and serum alkaline phosphatase. 4 In patients with kidney and liver disease the free fraction values of azapropazone correlated well with those of the anticoagulant drug phenprocoumon (r = 0.93, P<0