The expression of the human H1 zero histone gene and of a main type H1 gene was analyzed in two human leukemia cell lines. The main type, replication dependent H1 gene expression reflected the state of proliferation of both cell lines. No H1 zero mRNA was detected in the promyelocytic HL-60 line, whereas the monocytic U937 cells showed low steady-state levels of 1H zero mRNA. Stimulation of HL-60 with several known inducers of differentiation failed to induce any accumulation of H1 zero mRNA. Treatment of U937 with phorbol ester or butyrate, on the other hand, led to an increase of the H1 zero mRNA concentration.