In 30 patients with multiple myeloma who were scheduled for peripheral blood stem-cell transplantation, a quantitative analysis of the stem cells following enrichment by anti-CD34 was carried out. To detect the cells of the specific myeloma clone, polymerase chain reaction (PCR) was performed using unique allele-specific oligo primers for the immunoglobulin heavy chain rearrangement. The clonogenic cells before and after stem-cell enrichment, were quantified by a limiting dilution assay and a highly sensitive semi-nested PCR combined with a real-time quantitative PCR. In order to accomplish a statistically adequate end-point analysis, a large number of PCR analyses (40 per sample) were performed. By this technique the lowest detection limit observed was one myeloma cell per 106 cells. Myeloma cells were detected in 29/30 samples from the CD34-enriched fraction. The CD34 selection procedure resulted in a median 28-fold enrichment of CD34+ haemopoietic precursor cells. The stem-cell selection reduced the median concentration of clonal cells per million total cells by half, with a highly significant linear relationship between the number of myeloma cells before and after stem cell enrichment. The median depletion of clonal cells by the overall procedure was 2.15 log units, corresponding to a reduction of the total quantity of clonal cells reinfused into the patients by at least 99.3%. We conclude that CD34+ cell enrichment led to a reliable tumour cell depletion of the order of 2 log, which may not be sufficient since the total number of tumour cells in the leukapheresis product was 7.2 log (median).