The coculture effects of human oviductal cells on mouse embryo development in vitro were studied. Pronuclear stage mouse zygotes were cocultured with different cell types, or then cultured either in medium alone (control), spent medium from oviductal cell culture (conditioned medium) or high molecular weight fractions (> 10 and > 100 kDa) of the conditioned medium (reconstituted medium). Embryotrophic activities were compared between these groups in terms of percentage of morula and blastocyst formation, and cell count at the blastocyst stage. The mouse embryos developed better in oviductal cell coculture than in fibroblast coculture and medium alone culture. Conditioned medium and its reconstituted medium also provided a significant enhancement of embryo development in vitro when compared with the control medium culture, suggesting the production of high molecular weight embryotrophic factor(s) by the oviductal cells. The high molecular weight embryotrophic activity accumulated with the duration of conditioning could be serially diluted, and was abolished by heat or trypsin treatment. Replacing bovine serum albumin with polyvinyl alcohol in the culture medium did not affect the production of this high molecular weight embryotrophic activity by oviductal cells.