The role in early development of proteins secreted by oviduct epithelial cells has been investigated. Secreted proteins devoid of serum contamination have been produced by the surgical removal and immediate incubation of oviduct cells in [35S]methionine-containing medium. After electrophoretic separation, secreted polypeptides could be divided into those that were secreted uniformly throughout the oestrous cycle and a second class that showed a cyclical pattern of secretion. The first class of proteins represented a small proportion of total output whilst the predominant second class was composed mainly of polypeptides of Mr 92 and 46 x 10(3), respectively. Both of these polypeptide species, referred to as sheep oviduct proteins 92 and 46 (SOP 92, SOP 46), are detected only during the first 4 to 5 days after oestrus when the embryos are located in the oviduct. Oviduct cells collected at oestrus and maintained thereafter in culture secrete the same pattern of proteins and follow the same time course as their counterparts in vivo. The interaction between the oviduct proteins and the developing embryo was studied firstly by determining whether any of the secreted proteins bound to the zona pellucida. The results of iodination studies showed that two polypeptides of Mr 92 and 46 x 10(3), respectively, were bound to the zona pellucida of eggs removed from the oviduct but were absent from eggs that had not had contact with the oviduct epithelium. That these newly acquired proteins represent SOP 92 and 46 is suggested by their electrophoretic mobility and their ability to bind to the zona of follicular eggs when added in vitro and by the fact that they both disappear from the zonae of embryos after exit from the oviduct. The collection of unlabelled secreted proteins enabled us to produce a monoclonal antibody, which was used in the second series of experiments on oviduct-embryo interactions. The results confirmed that SOP 92 binds to the zona pellucida and moreover showed that this protein crosses the zona and becomes associated with the individual blastomeres of the developing embryo. These findings provide evidence that the mammalian oviduct probably plays a direct role in supporting embryonic development through specific polypeptides produced by its epithelium.