Hematopoietic cells, particularly megakaryoblastic ones, display a high level of low M-r phosphotyrosine protein phosphatase (LMW-PTP) expression; nevertheless, the role of this PTP in such cellular lineages has been scarcely investigated. Here, we demonstrate that LMW-PTP is able to associate and dephosphorylate signal transducer and activator of transcription-5 (STAT5) in DAMI megakaryocytic cells. Numerous researchers repeatedly hypothesized the association of a regulatory phosphotyrosine protein phosphatase with STAT5 C-terminus, but such phosphotyrosine protein phosphatase remained unknown. We show evidence indicating that the association of STAT5 and LMW-PTP does not exclusively involve the phosphatase active site and phosphotyrosine residue of STAT5, and we individuate an essential region of interaction at STAT5 C-terminus, coinciding with the previously hypothesized PTP-associating domain. (C) 2003 Elsevier Inc. All rights reserved.