Pigs are under consideration as possible sources of organs for xenotransplantation in humans. The induction of hematopoietic microchimerism through xenotransplantation of source animal hematopoietic cells has been suggested as a means to induce tolerance in potential recipients. Because all porcine cells contain genetic information for porcine endogenous retrovirus (PERV), coculture techniques, reverse transcriptase (RT) and reverse transcriptase-polymerase chain reaction assays were used to determine whether infectious PERV is released from fresh porcine bone marrow cells cultured in the presence or absence of porcine cytokines. Human embryonic kidney cell line, HEK-293 cells cocultured with porcine bone marrow cells were positive for PERV RNA but never became positive for viral RT activity, suggesting the PERV infection was not productive. In contrast, high levels of RT activity was detected in porcine ST-IOWA cells after coculture, demonstrating that these cells became productively infected. PERV was released from cultured porcine bone marrow cells without stimulation, and combinations of the porcine hematopoietic cytokines, interleukin-3, granulocyte macrophage-colony stimulating factor and stem cell factor had no additional effect on the infectivity or in vitro tropism of released PERV virions.