Early embryos assessed to be unsuitable for transfer to patients or for cryopreservation were cultured either in minimum essential medium (MEM) or in synthetic human tubal fluid (HFT). A significantly higher percentage of embryos developed to blastocysts in MEM (26.5%) than in HTF (14.5%). Serum was not required for blastocyst formation nor for hatching in MEM. HTF containing 1% human cord serum failed to support blastocyst hatching. Haploid (monopronucleate) and diploid (bipronucleate) embryos formed blastocysts and hatched with a similar frequency but triploid (tripronucleate) embryos developed poorly. Human chorionic gonadotrophin (HCG) released by hatched and intrazonal blastocysts was detected between days 7 and 8 after fertilization. The mean total HCG produced by hatched blastocysts by day 14 was 19,500 mIU, but embryos trapped inside their zonae released a mean level of 1550 mIU. Serum was shown to stimulate the output of HCG from both hatched and intrazonal blastocyst tissues. Free beta- and alpha-subunits of HCG were not released by developing blastocysts, but low levels of beta-HCG were produced by some embryonic tissues between days 12 and 14 when degenerative changes were observed.