Retinol, like retinoic acid, can induce differentiation of two embryonal carcinoma cell lines. These cells contain the specific retinol binding proteins in the cytosolic fractions. No correlation was found between the levels of binding activity and the effectiveness of retinol to induce differentiation. Microsomal fractions of embryonal carcinoma cells catalyse mannosyl transfer from GDP-mannose to retinyl phosphate. The formation of mannosyl retinyl phosphate appears a constitutive property and can be a mechanism by which retinoids induce early cell surface changes during differentiation of embryonal carcinoma cells. © 1983.