X‐linked agammaglobulinemia (XLA) is an immunodeficiency disease in man, resulting from an arrest in early B cell differentiation. The gene defective in XLA has recently been identified and encodes a cytoplasmic protein tyrosine kinase, named Bruton's tyrosine kinase (btk), essential for cell differentiation and proliferation at the transition from pre‐B to later B cell stages. In this study we investigated btk expression by Northern blotting experiments in a series of human (precursor‐) B cell lines, acute lymphoblastic leukemias and plasmacytomas, btk was found to be already expressed in very early stages of B cell differentiation, even prior to immunoglobulin (Ig) heavy (H) or light (L) chain gene rearrangements. Transcripts were also detected at the pre‐B cell stage and in mature B cells, irrespective of the Ig H chain class expressed. Approximately at the transition from mature B cells to plasma cells, expression of the btk gene is down‐regulated. In addition, the btk gene was found to be expressed in myeloid cell lines and acute myeloid leukemias. btk expression in myeloid cells is probably not a prerequisite for myeloid differentiation, since myeloid cells in XL A patients seem not to be affected. No btk expression was found in T‐lineage cells. The btk expression profile, i.e. from early precursor‐B cell stages preceding Ig rearrangement up to mature B cells, supports the hypothesis that the XLA defect resides in a critical step of B cell development which is independent of the Ig gene recombination machinery.