The expression of the low-affinity NGF receptor (p75) and the trkA proto-oncogene product was analyzed in a series of human hematopoietic cell lines at protein and RNA levels. We did not detect any form of NGF receptor in cell lines displaying a myelomonocytic phenotype (HL60 and U937). In contrast, cells displaying a more immature erythroleukemic phenotype (TF1 and K562) expressed TrkA in the absence of detectable p75. Scatchard analysis showed a single high-affinity site for NGF (kd = 10(-10) mol/L), with a copy number ranging from 300 to 3,000 sites per cell depending on the studied cell line. In addition, NGF induced autophosphorylation of TrkA and could substitute for granulocyte-monocyte colony-stimulating factor to trigger the proliferation of the TF1 cell line, with a half-maximal signal observed at 50 pmol/L, indicating that p75 is not required for DNA synthesis in this cell line. The physiologic relevance of NGF in early hematopoiesis was confirmed by showing that 12% to 15% of progenitor blood cells from mice treated with 5-fluorouracil expressed TrkA and that these cells could be induced to proliferate and differentiate in response to NGF in association with macrophage colony-stimulating factor. Our study demonstrates for the first time that trkA proto-oncogene expression and activation is not restricted to the nervous system, but is also an important element in early hematopoiesis.