The present study delineates the origin of the three major proteins constituting the rainbow trout (Oncorhynchus mykiss) zona radiata. Intraperitoneal administration of oestradiol-17 beta induced the appearance in the blood from juvenile fish (both sexes) of proteins immunoreactive to rabbit antisera against zona radiata proteins (zr proteins). These proteins had similar molecular weights to the zr proteins (alpha, 60 kDa; beta, 55 kDa; and gamma, 50 kDa). Primary hepatocyte cultures from fish treated with oestradiol-17 beta incorporated radioactive [35S] methionine into four major proteins with molecular weights of 160, 60, 55 and 50 kDa. Only the latter three proteins were specifically immunoprecipitated with antibodies to zr proteins. Furthermore, our data demonstrate that in such cultures the biosynthetic mole ratios of these secreted proteins (60, 55 and 50 kDa) are close to one. Control cultures from fish that had not been treated with oestradiol-17 beta failed to produce immunoreactive proteins. The data support the hypothesis that zr proteins are synthesized in a concerted manner in the liver during teleostean oogenesis and transported by the blood for deposition in ovarian follicles.