In previous studies, mouse placental lactogen I (mPL-I) and mPL-II were localized to trophoblast giant cells in the placenta at midpregnancy. The present study was undertaken to determine whether mPL-I and mPL-II are produced by two distinct populations of giant cells or by the same cells. A heterogeneous population of cells that included trophoblast giant cells was obtained by enzymatic dispersion and Percoll gradient centrifugation of placentas from days 7 and 9 of pregnancy. Cells from day 7 of pregnancy were cultured in serum-free medium for 5 days, and cells that contained mPL-I, mPL-II, or both mPL-I and mPL-II were identified by double-staining immunocytochemistry. The percentage of PL cells that contained both mPL-I and mPL-II increased from about 30% on the first day of culture to about 90% on the third, and then declined to zero by day 5. Between 50% and 60% of the PL cells contained only mPL-I on the first 2 days of culture, and then the percentage of PL cells containing only mPL-I declined. The percentage of cells that contained only mPL-II was low for 3 days (<10%) and then increased to about 80% of the PL-containing cells by day 5. Cells from day 9 of pregnancy were analyzed for the release of mPL-I and/or mPL-II by sequential reverse hemolytic plaque assay. Cells that released only one of the PLs, as well as those that released both PLs, were identified. A shift was present in the type of PL released by the cells when they were followed for two consecutive days of culture. On day 1, most of the plaque-forming cells released only mPL-I, but by day 2, the fraction of plaque-forming cells that released only mPL-I declined whereas the fraction that released only mPL-II increased. Cells that released only mPL-I on the first day of culture and both mPL-I and mPL-II or only mPL-II on the second day of culture were observed. These data suggest that under these culture conditions, PL cells follow a pathway in which they initially produce only mPL-I, then both mPL-I and mPL-II, and finally only mPL-II. In vivo, there is a shift at midpregnancy in the type of PL that is produced by the mouse placenta, and these data suggest that this shift results, at least partly, from a change in gene expression in one population of giant cells.