Summary. Blastocyst fluid was aspirated from Day 6\ m=1/ 2\ \ p=n-\ 7rabbit blastocysts and was added to cultures of granulosa cells obtained from preovulatory follicles of untreated rhesus monkeys or from follicles of monkeys treated with PMSG. The stimulation of progesterone secretion was measured and equated with that produced by hCG. The hCG-like activity was also measured in a radioreceptor assay using 125I-labelled hCG and porcine granulosa cells.In 8 out of 10 experiments with cultured cells from untreated monkeys, addition of 20% blastocyst fluid from Days 6\p=n-\9of culture stimulated progesterone secretion by 2\ x =r eq-\ to 6-fold. Similar findings were obtained in 5 experiments with cultures from PMSG\x=req-\ treated monkeys except that the blastocyst fluid was added from Days 0 to 6 of culture. The granulosa cells in such cultures underwent morphological luteinization. Compared to a standard of purified hCG the blastocyst fluid contained about 0\m=.\76\p=n-\2\m=.\5 ng hCG\x=req-\ like activity/ml which was non-dialysable. The radioreceptor assay indicated the presence of 0\ m=. \ 5\ p=n-\ 2\ m=. \ 5 ng hCG-like material/ml.