We have obtained and studied a 105,000-g pellet from T-3-C1-2 cells, a cloned line of Friend virus-induced erythroleukemia cells. By difference spectrophotometry, the pellet was shown to contain cytochrome b5 and cytochrome P-450, In the present study, we have used the Friend virus-induced erythroleukemia T-3-C1-2 line as a model for early erythroid cells to study cytochrome b5 , a protein found in reticulocytes and mature erythrocytes . In non-nucleated erythroid cells, cytochrome b5 (16, 31) and cytochrome b5 reductase (16,20,30,39) exist in the cytoplasm as soluble molecules . Erythrocyte cytochrome 65 is similar to cytochrome b5 solubilized from microsomes of the liver. The amino acid composition of bovine erythrocyte cytochrome b5 I is in very good agreement with a segment (residues 1-97) of bovine liver microsomal cytochrome b5 (6) . Tryptic digestions It is our contention that, in the immature erythroid cell, cytochrome b5 is associated with membranes, and that sometime during the maturation process this hydrophobic membrane-bound protein is converted to a water-soluble molecule . In this paper we will present evidence that cytochrome b5 in an immature erythroid cell system exists as a membrane-bound protein. Some of these results have been presented previously in abstract form (36) .