Indirect immunofluorescence was utilized to study the expression of chicken fetal antigens (CFA), developmentally phasic membrane antigens, on the primitive erythroid series during chick embryo development. Cells of this series expressed CFA determinants at all four stages of cellular maturation. However, the relative intensity of fluorescent staining (measurement of CFA determinants) varied with development. CFA determinant loss was correlated with the age of the embryo, and the stage of cellular maturation. Indirect immunofluorescence utilizing antisera capable of detecting eight individual CFA determinants revealed that all eight CFA determinants were present on early polychromatic erythrocytes of the primitive series, but that a selective and sequential loss of CFA determinants occurred in later stages. The data suggest that certain CFA determinants are differentially expressed during primitive erythroid cellular maturation in the developing chick embryo.