Serum, placental, extracts, and placental incubation medium from 12- and 17-day-pregnant mice were fractionated by gel exclusion chromatography on Sephadex G-100. The eluates were monitored for activity by a radioreceptor assay (RRA) for lactogenic hormones that detects placental lactogen (PL) and PRL. RRA activity in serum and placental extracts and incubation medium from both days of gestation eluted in two peaks, the first emerging in the void volume (Vo) and the second with an elution volume (Ve) to Vo ratio of 1.96, which was similar to the Ve to Vo ratio of a human PL marker. Fractions in the second peak stimulated casein synthesis in mouse mammary gland explants. The RRA activity of the eluates was not due to the presence of PRL, as determined by RIA, and therefore the activity in the second peak was identified as monomeric PL. Electrophoresis of mouse placental extracts and incubation medium from both days 12 and 17 of gestation on 10% polyacrylamide gels showed only one band of RRA activity (Rf = 0.22). The present results show there are no differences between the elution profiles on Sephadex G-100 and the electrophoretic mobilities of PL from serum, placental extracts, and placental incubation media from days 12 and 17 of pregnancy in mice.