Chromatin proteins from erythroleukemia cells transformed by Friend virus were analyzed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. A new chromosomal protein, apparent molecular weight 25,000, was shown to appear during erythroid differentiation of the cells induced by dimethyl sulfoxide or hexamethylenebisacetamide. This protein is tightly bound to the chromatin and does not appear to be related to any of the known histones. It is also demonstrated that this protein is not induced in a dimethyl sulfoxide-resistant variant of these cells.