ABSTRACT A method of haemagglutination inhibition assay of HCS was developed utilizing formaldehyde and glutaraldehyde to conjugate the hormone to sheep red blood cells. This technique is sufficiently sensitive (8.7 ng/ml) so that assay of HCS from early pregnancy serum (17 weeks of gestation) can be performed directly after 1:50 dilution. Results can be obtained within 3 h. HCS sensitized red blood cells were shown to be stable for several months. HCS levels of 20 normal pregnancies were estimated throughout gestation. Forty-two sera from pregnant subjects were assayed by this technique and radioimmunoassay (RIA). The values obtained by the 2 methods had excellent correlation (r = 0.95). Reproducibility of the present method was ± 15 %. Data of HCS serum levels obtained by the present technique and by RIA in high-risk pregnancies are compared and the conclusion is reached that any of these techniques can be used for the successful clinical assay of HCS.