A RIA for mouse placental lactogen-I (mPL-I) was developed using recombinant mPL-I as the standard, radioligand, and antigen for antiserum production. Displacement curves for dilutions of serum and placental extracts from pregnant mice were parallel to the recombinant mPL-I standard curve. Serum from male and nonpregnant female mice and high concentrations of mouse PRL, GH, PL-II, proliferin, and proliferin-related protein did not cross-react in the assay. mPL-I appeared in maternal serum on day 6 of pregnancy. Its concentration remained low until day 8 and them increased to a very large peak on days 9-11 (maximum concentration, approximately 8 micrograms/ml). The mPL-I concentration declined after day 11, but the hormone could be detected at low concentration in maternal serum until the end of pregnancy. On day 10 of pregnancy, the mPL-I concentration of maternal serum was correlated with litter size. Fractionation of serum from 10-day pregnant mice by size exclusion chromatography indicated the absence of high mol wt forms of mPL-I in the circulation.