Recent reports suggest that CD5+ B cells constitute up to 47% of the total B cells in normal peripheral blood (PB), a finding that would restrict the sensitivity of the CD5/CD19 flow cytometric assay for minimal residual disease (MRD) analysis in chronic lymphocytic leukemia (CLL). We studied 40 normal samples (PB, 20; bone marrow [BM], 20) using CD5-fluorescein isothiocyanate (FITC)/CD19-phycoerythrin (PE) immunostaining to evaluate the reference range of CD5+ B cells. The mean percentage of CD5+ B cells per total number of B cells was 12.2% (range, 3.6%-23.9%) in PB and 11.7% (range, 4.4%-19.5%) in BM. On serial dilution, this assay could detect 1 CLL cell in 1,000 leukocytes (sensitivity, 0. 1%). A distinct "bright" CD5+ B-cell subpopulation, consistent with a CLL-like-phenotype, was observed in 3 samples. Our results suggest that the CD5-FITC/CD19-PE assay has a clinically useful sensitivity for MRD analysis in CLL. The usefulness of this assay as a screening tool to identify the earliest stage of indolent CLL needs further study.