Ultrafiltration has been used to investigate the interaction of ascorbate with bovine serum albumin in 0.1 M sodium phosphate buffer, pH 6.5. The results are interpreted in terms of the binding of ascorbate to four equivalent and independent protein sites, governed by an intrinsic association constant of 2 600 +/- 700 M-1 at 20 degrees C, thereby providing evidence for specificity of the interaction over a range of vitamin concentration (0.08-1.5 mM) pertinent to the physiological situation.