Peripheral blood CD34+ hematopoietic cells (PBPC) mediate hematopoietic reconstitution in cancer patients after autologous transplantation and can be expanded ex vivo in the presence of colony-stimulating factors. This study shows that functionally active antigen-presenting cells (APC) for soluble proteins are generated and expanded in these PBPC cultures. CD34+ cells were cultured ex vivo in medium containing stem cell factor, interleukin-1 beta (IL-1 beta), IL-3, IL-6, and erythropoietin (EPO). The cells from these cultures developed into very potent APC of tetanus toxoid and purified derivative of tuberculin for autologous T cells in vitro. The antigen-presenting capacity of these cell was maintained for at least 38 days of culture. These APC resembled immature cells of the myelomonocytic cell lineage by surface marker, immunocytochemistry and ultrastructural analysis. Such APC might be able to present antigens from certain tumors to the immune system.