A new technique in which polyethylene glycol is added to double-antibody assays of lutropin and follitropin eliminates the need for an incubation period during the separatory phase. After the antigen-first-antibody equilibrium reaction, the second antibody and polyethylene glycol (30 g/L final dilution) are added, the tubes are immediately centrifuged, each supernate is aspirated, and the precipitated antibody-bound hormone is counted. With this combined polyethylene glycol-second-antibody separatory method, the incubation time is shorter than the usual double-antibody separation procedure, but avoids the problem of high nonspecific precipitation, which occurs with polyethylene glycol alone. In addition, the combined separation technique allows the use of sera containing low titers of second antibody and use of smaller volumes of sera containing high titers of second antibodies, thus conserving second-antibody reagent.