An unusual case of prolymphocytic leukemia of the B cell type (B-PLL) in a 79-year-old patient is reported. The clinical and cytomorphological features of the disease were typical of B-PLL, but membrane and cytoplasmic immunoglobulins (Ig) could not be demonstrated by immunofluorescence techniques; 3% to 4% of the cells were shown to have IgG kappa in the cytoplasm by a more sensitive immunoperoxidase method. The cells were unreactive with a panel of monoclonal antibodies against T cell antigens but they were positive with B cell lineage reagents: FMC4, anti-HLA-Dr determinants; FMC7, which reacts with most B-PLL; anti-B1 and anti-B4, which react with most B cell leukemias. Analysis of Ig genes at the DNA level demonstrated that both heavy- chain alleles and one kappa chain allele were rearranged, confirming that the patient's cells were of B lineage. Chromosome analysis revealed a consistent abnormality, t(17;21)(p11;p11), in all cells and, in addition, a 14q+ marker in 10% of the cells. This study highlights the value of DNA analysis techniques for the characterization of neoplastic B cells. The low rate of expression of Ig genes, despite their rearrangement, suggests that a specific transcriptional or posttranscriptional defect must exist in these cells.