Histidine-rich glycoprotein (HRG) is an alpha2-glycoprotein that was first described by Heimberger, et al, in 1972. Today, HRG is generally regarded as a mild prothrombotic protein. Blood samples of 585 individuals were collected with the aid of the Alameda-Contra Costa Medical Association (ACCMA) Blood Bank, Oakland, CA. Sex, age, ethnic origin, and blood-type information were available for each sample. The blood was processed to isolate the cell free plasma, and plasma HRG concentration was measured relative to that of a normal pool through a modified Laurell technique. Among Caucasian individuals, the mean HRG level of blood-type AB subjects, 125 +/- 28%, was found to be significantly greater than the means for subjects with A and O blood-types, 103 +/- 35% and 105 +/- 30% respectively (P = .0246). In addition, the average HRG level appears to increase linearly with age. The mean plasma level of HRG in subjects 50-59 years old was significantly greater than the level in subjects 30-39 years old (P = .0020). The correlation observed between blood-type and plasma HRG level in this study supports previously reported results that indicate significant genetic control over the plasma level of this protein. The age and blood-type based correlations observed in this study raise the question of whether these variables need be addressed if HRG level were to be employed in a clinical setting as a diagnostic tool.