It could be speculated for patients with myeloma and other lymphoproliferative disorders that peripheral blood stem cells may be preferable to bone marrow for autologous transplantation because they may be less contaminated by neoplastic cells. To test this possibility, the immunoglobulin heavy chain gene rearrangement and limiting dilution polymerase chain reaction were used to sensitively quantify myeloma cells in bone marrow and peripheral blood stem cell collections, taken at a similar time, from eight patients with multiple myeloma. Levels of residual disease in the peripheral blood stem cell harvests were variable and did not reflect the tumour burden in the marrow. Peripheral blood stem cells contained 17 to 23700-fold fewer myeloma cells compared with the bone marrow and would have resulted in reinfusion of 0.08 to 59480-fold fewer myeloma cells based on total reinfused CFU-GM and 0.24 to 24700-fold fewer myeloma cells based on total reinfused nucleated cells. Assuming that the proportion of clonogenic myeloma cells is equivalent, peripheral blood stem cells may be better than bone marrow as a source of haemopoietic stem cells for transplantation in multiple myeloma. The clinical followup suggested that patients transplanted with peripheral blood stem cells containing a low number of myeloma cells had better disease control than those transplanted with peripheral blood stem cells containing a high number.