Recent studies have focused on reproductive health of men in the general population. However, semen samples are difficult to obtain within sampling frames that allow comparisons. Blood samples are easier to obtain than ejaculates. Therefore, serum biomarkers of spermatogenesis are of major interest for population studies. FSH has previously been used as a marker of spermatogenesis, although it is also influenced by the hypothalamus. Serum inhibin B was recently suggested as a possible, more direct serum marker of spermatogenesis in men with testicular disorders. In a Danish nationwide collaborative study, we found an unexpected difference in semen concentration between two groups of men recruited from two different centres. We, therefore, analyzed reproductive hormones in blood, including inhibin B, to test whether the observed difference in semen concentration was reflected in the reproductive hormones. From 1992 to 1995, a total of 430 men, 20-35 yr old, who lived with a partner and who had not previously attempted to achieve a pregnancy, were recruited. The couples were enrolled into the study in one of two centres (centre A, n = 231; and centre B, n = 199) when they discontinued birth control. At enrollment, they provided a semen sample (n = 419), and a blood sample was drawn (n = 349). The semen analysis was performed in accordance with the WHO 1992 guidelines, and interlaboratory differences were tested. Inhibin B was measured in an enzyme immunometric assay, which has previously been described. All blood samples were analyzed in the same laboratory. Median sperm concentration and the percentage of morphologically normal spermatozoa were significantly higher among men from centre A (56.0 mill/mL and 42.5%), compared with men from centre B (44.8 mill/mL and 39%). Men from centre B had a significantly higher median FSH (3.42 IU/L) and a lower inhibin B (186 pg/mL) than men from centre A (3.21 IU/L and 209 pg/mL). The differences persisted after control for potentially confounding variables. A significant correlation was found between the cubic root-transformed serum FSH and inhibin B levels (r = -0.61, P < 0.001), between the cubic root-transformed serum FSH and sperm concentration (r = -0.40, P < 0.001), and between the cubic root-transformed inhibin B and sperm concentration (r = 0.38, P < 0.001). The predictive power of detecting sperm counts below 20 mill/mL among men who's inhibin B and FSH both were below 80 pg/mL and above 10 IU/L, respectively, was 100%. The unexpected significant difference in semen concentration between two groups of normal Danish men was probably caused by differences in sampling procedures in the two centres where the men were recruited, rather than geographical differences. However, similar differences in serum levels of inhibin B and FSH between centres were found. These findings suggest that a real difference in spermatogenic potential between the two groups of men existed. We suggest that serum inhibin B, in future population studies on male reproductive health, may serve as a new marker of spermatogenesis, in addition to sperm concentration and serum FSH.