SHIMIZU, H., HOSODA, K., MIZUMOTO, M., KUROKI, Y., SATO, H., KATAOKA, K., HAGISAWA, M., FUJIMOTO, S. and AKINO, T. Improved Immunoassay for the Determination of Surfactant Protein A (SP-A) in Human Amniotic Fluid. Tohoku J. Exp. Med., 1989, 157 (3), 269-278- A simple, improved immunoassay for the determination of human surfactant protein A (SP-A) in human amniotic fluid was developed. The immunoreaction with a monoclonal antibody PC6-immobilized plastic bead, peroxidase-labeled monoclonal antibody PE10 and amniotic fluid sample diluting in the buffer containing 0.6% sodium dodecyl sulfate/2% Triton X-100, was carried out simultaneously at 45°C for 30min in test tubes. After washing the bead with 2% skimmilk in phosphate buffered saline containing 1% Triton X-100, the peroxidase reaction was developed by adding the substrate reagent and the absorbance was measured. The amniotic fluid obtained at full term was used as a standard instead of purified SP-A, because of the stability of the antigenicity. This immunoassay method was used to measure SP-A in 69 samples of amniotic fluid from 22 to 41 weeks of gestation. The result indicated that the SP-A values obtained by the present immunoassay can be used for predicting the fetal lung maturity. This simplified monoclonal immunoassay completed the measurement within 1hr, and so it could be used routinely in clinical laboratory.