Summary The addition of pH 5 fraction from reticulocytes to a liver cell free system inhibits the synthesis of serum-albumin. The inhibitor, which is a protein, was partly purified by chromatography on CM- and DEAE-Cellulose. Its molecular weight is 60 000, its isoelectric point 5.4. This inhibitor has not effect on hemoglobin synthesis by a reticulocyte cell free system. We have previously isolated from liver pH 5 fraction by a similar procedure a protein which inhibits hemoglobin synthesis without affecting serumalbumin synthesis. The role of these inhibitors in cell differentiation is discussed.