The growth and maturation of haemopoietic cells is regulated by signal transduction through tyrosine protein kinases. Recently, a novel cytoplasmic tyrosine kinase gene in chromosome X, called Bmx, was identified in human bone marrow RNA. Bmx belongs to a subfamily of tyrosine kinases which are expressed in various haemopoietic cell lineages. We studied Bmx expression using RT-PCR of RNA from fractionated peripheral blood leucocytes, progenitor-enriched fractions of cord blood and from bone marrow or peripheral blood samples from leukaemia patients. Bmx was strongly expressed in haemopoietic tissues and enhanced in neutrophilic granulocytes. Bmx mRNA was also found in CD34-positive progenitor cells from cord blood. All samples (10/10) of patients with acute myeloid leukaemia and (4/4) with chronic myeloid leukaemia showed expression of Bmx. In contrast, none of the samples of acute lymphoid leukaemia (0/8) and only one out of six samples of chronic lymphoid leukaemia expressed Bmx. In conclusion, Bmx expression seems to be associated with myelopoiesis.