In multiple myeloma (MM), the presence of tumor cells in leukapheresis products (LP) has been demonstrated with highly sensitive molecular biological tools in up to 100% of cases. Therefore methods to reduce the tumor load of LP by CD34+ selection are envisaged. However, there is controversy as to whether the CD34+ cell is already involved in the malignant process. We have established a PCR assay with allele-specific oligonucleotide primers (ASO) complementary to the CDR3-hypervariable region of the immunoglobulin heavy chain gene of each patient's myeloma clone. Using this ASO-PCR, 43 LP of 10 patients with MM eligible for high-dose therapy were assessed for malignant cells. Furthermore, in an experimental setting we have examined 10 CD34+ and four CD19+ fractions obtained from PCR-positive LP by sequential preparative magnetic and fluorescence activated cell sorting (purity >96%) for the presence of the tumor-specific CDR3 region. The majority of LP harbored cells of the myeloma clone (93%), while all CD34+ fractions were PCR-negative. In all CD19+ fractions malignant cells were detected. These results confirm that CD34+ selection can be considered for LP in MM. The sensitivity of the ASO-PCR (up to 10[-5]) enables us further to monitor the efficacy of CD34+ enrichment protocols in the clinical setting.