The use of ex vivo expanded CD34-selected hematopoietic progenitor cells (HPCs) for autologous stem cell support or gene therapy is a major area of research and is likely to increase in the future. At present, little is known about the fate of contaminating malignant cells during ex vivo expansion of CD34-selected HPCs. We established a competitive polymerase chain reaction (PCR) titration assay to determine the number of residual lymphoma cells before and after selection and ex vivo expansion of CD34-selected HPCs in patients with t(14; 18) translocation carrying non-Hodgkin's lymphoma. Seven bone marrow (BM) and 2 mobilized peripheral blood progenitor cell samples from 8 patients without histologic BM involvement at the time of the harvest were analyzed by competitive PCR titration assay and determined to contain between < or = 10 and 4,000 lymphoma cells/ 10(6) mononuclear cells (MNCs). Immunoadsorption enriched CD34+ cells from a mean of 5% (range, 1% to 9%) to a mean of 88% (range, 76% to 94%) of MNCs and resulted in a 1 to 4 log depletion of contaminating tumor cells. Two HPC samples became PCR negative after CD34 selection, whereas 7 samples still contained < or = 10 to 200 residual lymphoma cells/10(5) MNCs. CD34-selected cells were consecutively expanded in suspension culture in the presence of stem cell factor, interleukin-1 beta (IL-1 beta), IL-3, and IL-6. The mean increase of cells was 13-fold (range, 4- to 22-fold) at day 7 and 65-fold (range, 43- to 110-fold) at day 14 of culture. Expansion resulted predominantly in myelomonocytic differentiation, whereas B-cell antigen-expressing cells became undetectable. Six of the seven PCR-positive CD34-selected samples became PCR-negative for the t(14; 18) translocation at day 7 and/or 14 of expansion. One PCR-positive and one PCR-negative CD34-selected sample were PCR-positive after ex vivo expansion, but the number of residual lymphoma cells remained at the limit of detection. We conclude that CD34-selection does not eliminate contaminating lymphoma cells in the majority of t(14; 18)+ HPC harvests. However, during ex vivo expansion of CD34-selected HPCs, residual t(14; 18)+ lymphoma cells do not proliferate and become undetectable by PCR in the majority of cases.