DNA, RNA, and/or protein cellular content were studied by flow cytometry in 52 cases of acute myeloid leukemia before and on day 4 of remission induction treatment. Bone marrow (BM) samples were stained after fixation by acridine orange for DNA and RNA content (37 cases) and by propidium iodide and fluorescein isothiocyanate for DNA and protein content (52 cases). A positive correlation was found between pretreatment protein content and BM blast involvement: the higher the percentage of blasts in BM smears the higher the mean protein content (p less than 0.05). Protein content was higher in monoblastic leukemia (M4 and M5) than in the granulocytic types (M1, M2, M3) (p less than 0.05). S + G2 + M was higher in patients with protein content below 80 arbitrary units than in the subgroup with protein content above this threshold (p less than 0.05). Pretreatment RNA content, estimated by the RNase-sensitive fraction of G1 cells, was significantly higher in undifferentiated and M1 leukemias than in the other cytological groups (p less than 0.0001). This fraction was higher in patients who subsequently achieved complete remission, but it was not related to BM blast involvement or proliferative fraction of cells. During cytostatic treatment the changes in RNA and protein content did not follow a typical pattern. The connections between variations of DNA, RNA, and protein content and prognosis are examined and their possible relation to drug-induced blast cell maturation is discussed.