Immunoreactive inhibin (i-inhibin) in the serum of the baboon fetus is high at midgestation and decreases toward term. Human cord serum also contains immunoreactive inhibin, but bioactive inhibin is nondetectable or very low. In the present study we report that baboon fetal serum is also inactive in the sheep anterior pituitary FSH bioassay. Furthermore, both fetal baboon serum and term human cord serum are inactive in two enzyme-linked immunosorbent assays (ELISAs) that use a monoclonal antibody (R-I) against the alpha-subunit of human inhibin as the capturing agent and a monoclonal antibody (E-4) against the beta A-subunit as a detection/quantification agent (ELISA-A), or the E-4 antibody as the capturing agent and a R-1 F(ab) fragment as the detection/quantification agent (ELISA-B). Recombinant human inhibin A was reactive in both ELISAs. Human cord serum inhibited recombinant human inhibin activity in the ELISA-A, but not in the ELISA-B. An immunoaffinity column to which the R-1 antibody had been coupled extracted i-inhibin activity from both baboon fetal heart serum and human cord serum. Together, these results suggest that the i-inhibin in fetal serum is the free alpha-subunit, with little, if any, dimeric inhibin present. Western blot analysis confirmed that the i-inhibin activity extracted by the R-1 immunoaffinity column from baboon fetal serum can be attributed to a free alpha-subunit(s).