Erythroleukemic cells transformed by the AEV or S13 strains of avian erythroblastosis virus differentiate in vitro either spontaneously (S13) or following a temperature induction (temperature-sensitive mutants of AEV). To study differentiation in these cells at the molecular level, homogeneous fractions of maturing cells at discrete stages of differentiation were prepared by Percoll density-gradient centrifugation. This method was also used for the fractionation of differentiating normal erythroid cells separated from total bone marrow by an immunological "panning" technique. Total protein synthesis in these cells was then analyzed by two-dimensional gel electrophoresis. The expression of several proteins was altered in differentiating leukemic cells but not in their normal counterparts. However, in general, the normal and leukemic cells from comparable stages of maturity showed closely related protein synthetic patterns. Similar early and late changes in the synthesis of a number of polypeptides were detected during maturation from early erythroid precursors to terminally differentiated erythrocytes. Further, the leukemic as well as the normal cells appeared to undergo a major switch in total protein synthetic pattern during late differentiation. These results demonstrate that normal and erythroleukemic cells differentiate along similar pathways.