Expression of human fos proto-oncogene (c-fos) was analyzed in primary cells from 50 untreated acute lymphocytic (ALL) and myeloblastic (AML) leukemias. c-fos RNA, analyzed by blot hybridization, was detected virtually only in myelomonocytic (M4) and monocytic (M5) AML. Both M4 and M5 samples show a strong positive correlation between the amount of c-fos transcripts and the percentage of leukemic cells expressing surface antigens specific for mature monocytes and macrophages. Normal mature monocytes exhibit a detectable level of c-fos RNA, which is virtually unaltered on activation to macrophage differentiation, but is always below that observed in M4 through M5 monocyticlike cells. These data provide evidence that c-fos expression is linked to terminal monocyte and macrophage differentiation in normal and leukemic hemopoiesis.