A 29 kD protein previously isolated from murine erythroleukemia (MEL) cells and shown to enhance the rate of differentiation of these cells has now been demonstrated to possess an amino acid sequence identical to that reported for the 30 kD heparin-binding protein from developing rat brain, named amphoterin after its highly dipolar structure. The identity between the two proteins has been established on the basis of a strong heparin binding affinity and a complete homology in the amino acid sequences of N-terminal region as well as of several tryptic peptides. Furthermore, the cDNA encoding this protein has been isolated from MEL cell mRNA, by means of reverse transcriptase-polymerase chain reaction, and its sequence was found to correspond to that of amphoterin. The MEL cell differentiation enhancing factor, previously abbreviated as DEF, is again confirmed to reduce the latent period preceding the appearance of hexamethylenebisacetamide induced cell commitment and to stimulate the catalytic activity of alpha-protein kinase C. Thus, here we demonstrate that a protein expressed in MEL cells, whose sequence is identical to that previously reported for amphoterin, plays an essential role in promoting cell differentiation, thereby indicating a new relevant function of amphoterin.