In a group of 76 clinically healthy human volunteers, we found a high correlation (r = 0.997, p less than 0.001) between the ceruloplasmin oxidase activities in serum (mean 127.2 U/l) and plasma (mean 124.0 U/l). Similarly high correlation was observed for immunoreactive ceruloplasmin, which was assayed in 65 samples (r = 0.993, p less than 0.001), with means of 306.1 mg/l in serum and 296.4 mg/l in plasma. Although the difference between the means measured in serum and plasma is in both cases statistically significant, it is clinically irrelevant according to the criteria of the College of American Pathologists. Finally, a high correlation between serum and plasma concentrations (r = 0.970, p less than 0.001, N = 32) was also found for acid alpha 1-glycoprotein, whose sialic acid content is greater than that of ceruloplasmin; in this case there was no statistically significant difference between the means. These results suggest that in humans, ceruloplasmin does not bind to erythrocyte and/or platelet membranes via sialyl groups during clotting, which disagrees with the hypothesis of Paynter (Aust. J. Biol. Sci. (1982) 35, 353--361) in cows and sheep.