AbstractCD45, a transmembrane tyrosine phosphatase, is found on almost all nucleated hematopoietic cells and plays a crucial role in lymphocyte activation and differentiation. We recently achieved isolation of the human LSM‐1 (hLSM‐1) gene, whose product is a possible substrate for CD45, and we raised antibodies against the hLSM‐1 protein. hLSM‐1 expression in hematopoietic cells was examined with Northern and Western blot, fluorescence‐activated cell sorter, and immunocytochemical analyses. It was found that in the lymphoid lineage, T and B lymphocytes as well as NK cells expressed LSM‐1, whereas terminally differentiated plasma cells did not. As for the myeloid lineage, immature myeloid cells expressed LSM‐1, whereas terminally differentiated granulocytes and monocytes did not. In the erythroid lineage, normal erythroblasts expressed very low levels of LSM‐1, while erythroid cell lines (K562 and HEL) did not. Megakaryocytes did not express LSM‐1. Both CD34+/CD33−and CD34+/CD33+hematopoietic progenitor cells weakly expressed LSM‐1. These results showed that LSM‐1 is expressed in a lineage‐ and differentiation stage‐specific fashion. Am. J. Hematol. 54:1–11