The purpose of the study was to determine the normal values of Willebrandt factor antigen (WF:Ag) in accordance with AB0 blood group affiliation and to compare the results of its determination when two test systems were used. Ninety-four plasma samples from healthy individuals aged 18 to 62 years (median 38) were tested. The test systems manufactured by Technoclone and Axis-Shield were employed. In the samples from blood group 0 subjects, the level of WF:Ag was significantly lower than that in those from blood group A, B, and AB subjects (p 0.05). Great discrepancies were found in the results of some tests when WF:Ag was determined using various test systems. This is accountable to different specificity of the anti-WF antibodies obtained by various manufacturers. Nevertheless, the revealed differences, the lower normal ranges calculated from the results obtained using these test systems agree and no great differences were found between the samples (p > 0.05). Therefore, both test systems have the equal value in the diagnosis of Willebrandt disease.