A sensitive and specific enzyme-linked immunosorbent assay (ELISA) suitable for measuring rat and human luteinizing hormone (LH) is described. The LH-ELISA used anti-bLH β subunit monoclonal antibody-coated plates, an antiserum against rLH or hLH and an antibody against rabbit IgG labelled with peroxidase. Using rLH-RP-3 or hLH WHO IRP 68/40 as standard diluted in assay buffer, the LH-ELISA had a sensitivity of 50 pg/ml and 0.78 mlU/ml, respectively. The LH-ELISA allowed accurate determination of LH in buffer, cell culture media, anterior pituitary extracts and sera and was highly specific for rat and human LH. The use of this ELISA offers improvements in convenience, economy, sensitivity and safety over comparable radioimmunoassay procedures.