Chloroquine protein binding was determined by equilibrium dialysis of purified plasma proteins and plasma samples from 20 healthy subjects and 14 patients with rheumatoid arthritis. The mean binding was 61 +/- 9% in plasma from healthy subjects (range 46-74%) and 64 +/- 7% in plasma from rheumatoid arthritis patients (range 55-79%). Albumin and alpha 1-acid glycoprotein at physiological concentrations bound chloroquine to an approximately equal extent. Protein binding is unlikely to be an important determinant of chloroquine pharmacokinetics or response.