The malignant monoclonal population in hairy cell leukemia (HCL) has been variously ascribed to be of myeloid, B, or even T cell origin. Recent data have been interpreted as suggesting that hairy cells (HC) may concomitantly or serially express both B and T surface determinants, a phenomenon which, if verified, would be unique among the lymphoproliferative malignancies. Data described here, however, demonstrate that (1) at least the majority of HCL are phenotypically of B cell derivation, and (2) the initial B cell phenotype is retained and solely expressed on cultured as well as phytohemagglutinin (PHA) activated monoclonal malignant HC.

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