The murine sarcoma virus 3611 contains the transforming v-raf gene that has partial nucleotide homology with the src family of tyrosine kinase-encoding oncogenes. Although this virus induces fibrosarcomas in mice, a recombinant murine retrovirus carrying both the raf and myc oncogenes induces immunoblastic lymphomas and immortalizes mouse macrophages in vitro. The present study has thus monitored the expression of c-raf in human hematopoietic cells. The results demonstrate the presence of a 3.6-kb c-raf transcript in HL-60 promyelocytic leukemic cells. The induction of HL-60 cell differentiation along the monocytic or granulocytic lineages had no detectable effect on the level of c-raf transcripts. Furthermore, in contrast to c-myc and c-fms expression, inhibition of protein synthesis with cycloheximide had no detectable effect on c-raf expression. Similar levels of c-rafRNA were also found in other human cell lines derived from myeloid, B cell, and T cell tumors, as well as in normal granulocytes, monocytes, and macrophages. These findings suggest that the c-raf protooncogene is widely expressed in multiple hematopoietic lineages.