A monoclonal antibody specific for the chicken transferrin receptor was used to study receptor expression on circulating red cells from chick embryos of different ages. The use of indirect immunofluorescence with this antibody showed that all circulating immature reticulocytes and primitive series erythrocytes--but not erythrocytes from the definitive series--expressed the receptor. In all cells, the protein was synthesized as a 90-95-kD form. The retention of the transferrin receptor (and another proliferation-dependent cell surface protein) contrasted with the behaviour of a series of other developmentally regulated antigens which are lost during maturation of both primitive and definitive series erythroid cells.