To evaluate relationships between c-fos proto-oncogene expression and specific lineages of hematopoietic differentiation we analyzed the constitutive and TPA-induced expression of the c-fos gene in a wide variety of fresh human leukemic cells. High constitutive c-fos expression was detected in acute leukemias with monocytic phenotype (FAB M4/M5) and in subsets of B lymphoid leukemias, some of which coexpressed B lymphocytic and monocytic markers. Conversely, low basal levels of c-fos transcripts were found in pure acute granulocytic leukemias (FAB M1/M2/M3), in erythroleukemias (FAB M6), in the great majority of B, and in all T lymphoid leukemias. TPA-induced c-fos expression seems to correlate with monocytoid differentiation only when sustained levels of transcripts (ie, detectable for at least 24 hours) were detected. Sustained c-fos expression was in fact observed only in those myeloid or lymphoid cells that acquired a stable monocyte-like phenotype in response to the phorbol ester. These results indicate that high constitutive c-fos expression may identify myelomonocytic-oriented forms of leukemia, specific subsets of B lymphoid malignancies, and at least some cells terminally differentiated in vitro to a monocyte-like phenotype. c-fos oncogene expression can therefore be regarded as an additional marker for the subclassification of human leukemias.