Four distinct alpha-fetoprotein (AFP) species were separated by ion-exchange chromatography of the approximately 70 000 dalton fraction of an extract of late-gestation fetal mice. Although chromatographicaly heterogeneous, antigenic differences among the AFP species resolved were not detected using classical double-diffusion methods. Analysis of the AFP contribution to total body saline-extractable protein of fetal and neonatal mice revealed an abrupt decrease in the fetal AFP contribution to total body saline-extractable protein commencing after the 18th day of gestation.