Insulin-like growth factor-binding protein-1 (IGFBP-1) has been purified from amniotic fluid by anion exchange, hydrophobic interaction and gel filtration chromatography. The overall recovery of the purification process was 12.2%. The purified IGFBP-1 yielded a single band on SDS-PAGE gel but showed two bands (34 kDa and 68 kDa) on Western blot under non-reducing conditions. Polyclonal antisera were raised by immunization of sheep using the purified IGFBP-1. The best antiserum bound 50% of 125I-labelled IGFBP-1 at a final dilution of 1:500,000. A radioimmunoassay for IGFBP-1 was developed. This assay had a minimum detection limit of 5 micrograms/l, and was used to determine serum levels in non-pregnant and pregnant women. There was no cross-reaction with a wide variety of materials tested. Serum IGFBP-1 levels in non-pregnant individuals (33 +/- 16 (S.D.) micrograms/l) were found to be significantly lower than those in the second (96 +/- 64 micrograms/l) and third trimesters (95 +/- 60 micrograms/l) of pregnant women. During pregnancy, circulating IGFBP-1 levels increased rapidly in the first trimester and reached a peak at 12-13 weeks of gestation (107 +/- 75 micrograms/l). The level then remained at 80 +/- 53 to 103 +/- 70 micrograms/l until term.