Alpha-fetoprotein (AFP) is a tumor marker for hepatomas and germ cell tumors, and the serum concentration has prognostic significance in other diseases. We examined the normal serum concentration of AFP in adults and sources of variation in the immunochemical variation of AFP. The serum concentration of the tumor marker alpha-fetoprotein (S-AFP) was log-normally distributed in 284 adult blood donors. S-AFP increased with age (p < 10^-7), whereas no gender-related difference was found. Reference intervals (95-interpercentile) were constructed for persons ≤40 years (0.60-9.30kIU/L) and >40 years (1.40-12.60 kIU/L). The concentration of AFP was significantly, albeit slightly, higher in serum than in plasma, whereas hemolysis, pretreatment with KCl and food intake did not influence S-AFP. S-AFP only changed 6% when measured twice 2 months apart (p = 0.04). Three enzyme immunoassays, using three different anti-AFP monoclonal antibodies for detection, were compared and two assays gave S-AFP values significantly higher, 2.8% (p = 0.03) and 19.0% (p < 10^-4), than the other assay. Thus, the choice of antibody may influence the result of immunochemical concentration determination. This can be explained by the existence of conformational variants of AFP with different antibody reactivities, and calls for careful standardization of monoclonal antibodies used in assays for AFP. With broad population reference ranges and slight intra-personal variation, the most effective reference range for S-AFP is previous values obtained in the same person.