Abstract: The present invention relates to a process for producing a transfusable, oxygenating composition of human red blood cells by the ex vivo culturing, expansion and differentiation of human primitive hematopoietic cells. The process involves expansion of primitive hematopoietic cells in a first bioreactor containing one or more growth factors, differentiating the cells into erythroid progenitor cells in a second bioreactor containing one or more differentiation factors and effecting maturation of the erythroid progenitor cells into mature erythrocytes in a third bioreactor containing one or more maturation factors. The invention also provides a process for producing a transfusable, oxygenating composition of red blood cells by expansion and differentiation of primitive hematopoietic cells in a bioreactor containing a nutrient medium which includes perfluorocarbons which improve the exchange of oxygen and carbon dioxide between the cells and the nutrient media.