A monoclonal antibody raised against ovarian follicles of the stick insect Carausius morosus reacted with two related polypeptides of 157 and 85 kDa in both the ovary and the hemolymph. In vitro cultured fat body proved capable of releasing the 157-kDa polypeptide into the culture medium and processing it to the lower-molecular-weight polypeptide of 85 kDa. This was further demonstrated by in vitro exposure to [35S]methionine. Under the same culturing conditions, ovarian follicles proved incapable of synthesizing and/or secreting the 85-kDa polypeptide. However, in vivo [35S]methionine-labeled ovarian follicles released both polypeptides when cultured in vitro for up to 24 hr. Vitellogenin polypeptides were labeled in vivo following exposure to [3H]glucosamine, while 157- and 85-kDa polypeptides were labeled only in ovarian follicles exposed in vivo to sodium [35S]sulfate. Under in vitro conditions, the 157-kDa polypeptide could be labeled with sodium [35S]sulfate only if ovarian follicles were cocultured with fat body. No sulfation occurred in fat body or ovarian follicles cultured separately. These experiments suggest that the 157-kDa polypeptide is a fat body-derived polypeptide that is sulfated upon transfer to the ovarian follicle.