Signal transducer and activator of transcription-5 (STAT5) plays an important role in repopulating activity of hematopoietic stem cells (HSCs). However, the relationship of STAT5 activation with early acting cytokine receptors is not well established. We have directly compared bone marrow (BM) from mice mutant for STAT5a and STAT5b (STAT5ab(-/-)) with that from mice lacking c-Mpl (c-Mpl(-/-)), the thrombopoietin receptor. Both STAT5 and c-Mpl deficiency only mildly affected committed myeloid progenitors assayed in vitro, but STAT5ab(-/-) BM showed lower Gr-1+ (4.4-fold), B220+ (23-fold), CD4+ (20-fold), and Ter119+ (17-fold) peripheral blood repopulating activity than c-Mpl(-/-) BM against wild-type competitor in long-term repopulating assays in vivo. Direct head-to-head competitions of STAT5ab(-/-) BM and c-Mpl(-/-) BM showed up to a 25-fold reduction in STAT5ab(-/-) contribution. Differences affecting reconstitution of primitive c-Kit+Lin-Sca-1+ multipotent progenitor (MPP)/HSC (1.8-fold) and c-Kit+Lin-Sca-1- oligopotent progenitor BM fractions (3.3-fold) were more modest. In serial transplantation experiments, STAT5ab(-/-) and c-Mpl(-/-) BM both failed to provide consistent engraftment in tertiary hosts and could not radioprotect lethally irradiated quaternary recipients. These results indicate substantial overlap in c-Mpl-STAT5 signaling defects at the MPP/HSC level but indicate that STAT5 is activated independent of c-Mpl to promote multilineage hematopoietic differentiation.