Protein binding of urate may have some pertinence to the pathogenesis of gout. However, binding studies have been hampered by problems with in vitro methodology and by the problem of relating the results of in vitro studies to the physiological situation. In the present study urate binding was determined by an ultrafiltration procedure. All manipulations were performed anaerobically and at 37 degrees in order to maintain the sample under physiological conditions. Normal urate bound was 15 +/- 7.5% in males and 10 +/- 6.6% in females. Urate binding did not correlate significantly with either the albumin or total urate concentration. It is suggested that the interaction between protein and urate is influenced by a number of factors other than the concentrations of free urate and binding protein. Some possible ones are discussed. The techniques described might usefully be applied to a study of urate binding in various pathological states.