The expression of the c-fos protooncogene was investigated by in situ hybridization in normal murine bone marrow cells. A strong signal was found in murine marrow cells having the morphologic features of erythroblasts. This result was confirmed in human marrow cells using a double labeling technique (in situ hybridization and immunocytochemistry). A majority (70%) of the cells expressing c-fos mRNA were glycophorin A-positive. In contrast, granulocytic precursors (CD 15-positive) or monocytes and their precursors (CD 14-positive cells) did not significantly hybridize with the c-fos probe. In addition, c-fos mRNA (2.2Kb) was detected by Northern blotting in RNA extracted from homogeneous populations of erythroblasts obtained by immune panning from fetal liver and from adult blood BFU-E-derived colonies. Fos protein was also detected in erythroblasts by immunofluorescence. The high level of c-fos mRNA previously found in hematopoietic tissue should therefore be related to the transcription of the c-fos gene during terminal erythroid differentiation.