In humans CD34 is a valid and reliable marker for hematopoletic stem and progenitor cells. In general, solid tumors, with the exception of endothelial cancers, do not express CD34. Accordingly, immunological selection of CD34+ hematopoietic stem/progenitor cells can be used to remove CD34- malignant cells in the setting of autotransplantation. To rule out CD34 expression on tumor cells from small cell lung cancer (SCLC), eleven SCLC cell lines were analyzed by flow cytometry. Interestingly, two of these were positive for CD34 and their expression of full-length CD34 was further confirmed by reverse transcriptase and polymerase chain reaction (RT-PCR). This finding indicates that prior to subjecting SCLC patients to the above treatment modality, preparing CD34+ hematopoietic stem/progenitor cells from SCLC patients for autotransplantation, CD34 expression on their tumor cells should be screened using immunohistochemistry and/or flow cytometry.