A two-site immunofluorometric assay for human placental lactogen.

A two-site immunofluorometric assay for human placental lactogen (HPL) in serum has been developed. Samples and standards are incubated for 10 min with an excess of sheep anti-HPL serum covalently coupled to particles of magnetisable cellulose. After sedimenting the particles and adsorbed hormone on a magnet the supernates are aspirated (or decanted) to waste. An excess of purified sheep anti-HPL immunoglobulin, labelled with fluorescein, is added and, after a further 20 min, the fluorescence remaining in the supernates, after sedimenting the particles, is inversely related to the initial concentration of HPL. Results correlate closely with those of an established radioimmunoassay (r = 0.92), within- and between-assay coefficients of variation are less than 10% and, employing a 200 microliter sample volume, the assay extends from a minimum detection limit of 0.02 mg/l throughout the entire range of values encountered in pregnancy to more than 10 mg/l.