It has recently been reported that human thrombopoietin (TPO) acts on early hematopoietic progenitor cells. Consequently, we investigated the effects of TPO on murine hematopoietic progenitor cells using lineage negative (Lin-), Sca-1+, c-Kit+ marrow cells from 5-fluorouracil-treated mice. One hundred enriched cells were cultured in suspension with various single cytokines for 5 days. When cultured with the single cytokines as stem cell factor (SCF), TPO, or interleukin (IL)-3, these cells were maintained or had increased by day 5, whereas only a few cells survived in cultures with granulocyte colony stimulating factor, IL-11, or IL-6. We extended the study in serum-free or serum-containing suspension cultures with SCF or TPO. Anti-TPO antibodies did not inhibit the effects of SCF on enriched cells but did inhibit the effects of TPO on those cells. We further examined the effects of TPO, SCF, and IL-3 on other populations of murine hematopoietic progenitor cells. Either TPO or SCF as a single cytokine could maintain murine Lin-, Sca-1+, c-Kit+, CD34- marrow cells, which are the most dormant cells. In addition, IL-3 increased Lin-, Sca-1-, c-Kit+ cells more than did SCF and TPO but did not stimulate Lin-, Sca-1+, c-Kit+, CD34- cells more. These results indicate that TPO as well as SCF may be key regulators in the proliferation of murine hematopoietic early progenitor cells.