We describe a simple and reliable radioimmunoassay (RIA) method for the measurement of human placental lactogen (hPL) in undiluted serum. Standard curves, in serum, cover the range of 0.015 to 1 mlU/L for early pregnancy monitoring (7 to 15 weeks, referred to as Procedure I), and 0.531 to 17 mIU/L for mid and late pregnancy monitoring (16 to 40 weeks, referred to as Procedure II). In a typical assay, 0.05 mL of serum are incubated with constant amounts of antiserum and 125I-hPL for 1 hour (in Procedure I), or 30 minutes (in Procedure II). At the end of the incubation, a 260g/L polyethylene glycol solution is added, in order to bring the polymer concentration in the assay tube to 130g/L. The tube contents are mixed and centrifuged. The supernatant is removed and the precipitate counted. The antiserum dilution for both procedures was chosen after classical titration without and with addition of 0.531 mIU/L of unlabelled hPL; the titration was performed with the desired incubation time of 30 minutes. This technique showed a good correlation with a solid phase second antibody RIA (r = 0,95; n = 215).