The assay of human chorionic gonadotropin and luteinising hormone by reverse passive haemagglutination reaction, using monoclonal antibodies coupled to red cells, is described. Quantitation is achieved by end-point determination for serial dilutions of standard or sample, the haemagglutination reaction being observed after settling under gravity for 90 min. Red cell-labelled antibodies were stabilised with glutaraldehyde without loss of sensitivity and allowing long term storage. Various antibody combinations were assessed, and the best combination under optimum conditions gave a positive haemagglutination reaction down to 0.2 ng/ml with HCG.