The expression of CD34 antigen on the surface of bone marrow (BM) cells during remission induction was studied in 20 patients with CD34-negative acute myeloblastic leukemia (AML). The patients were given high-dose methylprednisolone (HDMP) alone for one week, after which time mitoxantrone and low-dose Ara-C were added. BM cells from all patients were studied one, two and four weeks after initiation of treatment to evaluate CD34 antigen expression using a three-step peroxidase antiperoxidase staining technique. The mean percentage of CD34-positive BM cells was 5.3% at presentation, increasing to 15.6% in the first week, 12.9% in the second week and 21.7% in the fourth week of therapy. During the same period the mean percentages of the initial BM blasts decreased from 64% to 22%, 7% and 2% in the first, second and fourth weeks of therapy, respectively. The increase in the CD34-positive BM cells one week after HDMP treatment alone suggests that HDMP directly or indirectly stimulates CD34-positive hematopoietic progenitor cells while decreasing BM blasts in patients with AML.