During the ontogenic change from fetal to adult human erythrocytes, as well as fetal haemoglobin being replaced by adult haemoglobin, the cell-surface antigen i is converted to I (ref. 1). Recently it has been shown that this antigenic change is the conversion of the linear repeating Gal beta 1 leads to 4GlcNac beta 1 leads to 3Gal structure to branched Gal beta 1 leads to 4GlcNac beta 1 leads to 3(Gal beta 1 leads to 4GlcNac beta 1 leads to 6)Gal structure. We have shown that cell-surface labelling followed by endo-beta-galactosidase digestion can distinguish these two forms on the cell surface, and that band 3 and band 4.5 are the major carriers for these antigens on mature erythrocytes. Human leukaemic cell line K562, originally isolated from a patient at blast crisis of chronic myelocytic leukaemia, has recently been shown to synthesize glycophorin A, and to be capable of synthesizing haemoglobin upon induction. I demonstrate here that K562 cells express the fetal type (i) antigen on distinctly different glycoproteins from those of erythrocytes, by the use of cell-surface labelling followed by endo-beta-galactosidase digestion or followed by immunoprecipitation with specific antibodies.