The major sialoglycoproteins or glycophorins of the murine erythrocyte membrane, gp2 (Mr 44,000) and gp3 (Mr 29,000), are expressed as identical or closely related antigens by the three erythroid cell lines that succeed each other in normal mouse development. The embryonic forms of gp2 and gp3 differ from the adult forms by their mobility on SDS-PAGE. The apparent Mr values are increased by 1000-2000 for gp2 and 500 for gp3. An increase in the microheterogeneity of both embryonic and fetal forms of gp2 is also detected. Variations in glycosylation account in part for the apparent differences in Mr. Sizing of O-glycosidically linked oligosaccharide chains reveals that fetal glycophorins contain predominantly trisaccharide units while their adult counterparts are mostly tetrasaccharides. The kinetics of gp2 and gp3 biosynthesis in fetal liver are comparable to those established for the splenic erythroblasts of adult anemic mice. Like their adult counterparts, fetal glycophorins incorporate [3H]palmitate and [3H]galactose. The results indicate that, in murine ontogeny, distinct but antigenically related sialoglycoproteins are produced at each erythropoietic stage.