The rabbit immunoglobulins of the class IgG and IgM, but not IgA, have previously been shown to be transmitted to the foetal blood across a foetal membrane called the yolk sac splanchnopleur when injected into the uterine lumen. However, it has been claimed that IgM, unlike IgG, is absent from the uterine fluid and that the yolk sac splanchnopleur is not the natural route for IgM maternal-foetal transport. The present investigations utilised immunodiffusion and immunofluorescence techniques in order to detect endogenous immunoglobulins in sera and fluids and to localise them in the yolk sac splanchnopleur and uterine tissues. Contrary to previous claims, IgM, as well as IgG, has been detected in uterine fluid from 18 days of pregnancy onwards and IgM shown to have a localisation within the yolk sac splanchnopleur similar to that of IgG and indicative of its transport across the endodermal cells to the vitelline circulation. IgA could not be detected in the yolk sac and unlike IgM and IgG was absent from the foetal serum, but present in the uterine fluid. lmmunofluorescence studies on the uterine tissue highlighted the importance of the degenerating decidual tissue in the transport of IgM and IgG to the uterine fluid. Transport across the uterine epithelium and into the uterine glands appears to make little if any contribution as deduced from this technique. Plasma cells secreting IgM and IgA but not IgG have been detected within the endometrial stroma. It is suggested that their function is moreto supply local intrauterine immunity rather than to confer passive immunity on the foetus. Experiments in which human IgM was injected into the maternaI circulation and uterine lumen confirmed the importance of the decidual tissue in the transport of IgM to the uterine fluid and that the yolk sac splanchnopleur was a site of transport of IgM to the foetus. Preliminary investigations attempting to study receptor mediated transport of an HRP-HigM conjugate across the rabbit yolk sac splanchnopleur in vitro were unsuccessful probably due to poor preservation of the tissue.