Monoclonal plasma cells (CD38+CD45-/dim) are typically present in the blood of patients with active myeloma and can contaminate stem cell harvests. This has led to strategies that select CD34+ cells for use in autologous stem cell transplantation with the goal of decreasing tumor cell contamination. The aim of this study was to learn if the CD34 antigen is expressed on monoclonal plasma cells in the blood or marrow of patients with multiple myeloma. We used three-color flow cytometry (surface CD38;CD45 and cytoplasmic kappa or lambda) to identify monoclonal plasma cells in the blood (n = 24) and marrow (n = 37) from patients with plasma cell proliferative disorders. In each case the CD38+CD45- and CD38+CD45dim+ monoclonal populations were then analyzed for CD34 expression. In all 24 blood and 37 marrow samples, the CD38+CD45-monoclonal plasma cells were negative for CD34 expression. CD38+CD45dim+ monoclonal cells were documented in the blood of 11 patients and in the marrow of 33 patients and this cell population was also CD34-negative in all cases. These results indicate that CD34 is usually not expressed on the CD38+CD45-CD45dim+ monoclonal plasma cells in the blood or marrow of patients with plasma cell proliferative disorders. CD34 selection methods should therefore decrease the chance of tumor cell contamination of the stem cell product.