The methylation state of CCGG sites in and around the human ornithine decarboxylase gene, oncogenes c-myc and erb-A1, and actin genes were determined in human malignant leucocytes from patients with acute and chronic myeloid leukemia, chronic lymphatic leukemia, polycythemia vera, and multiple myeloma by means of isoschizomeric restriction endonuclease analysis. When compared with DNA from leucocytes of healthy controls, the ornithine decarboxylase and erb-A1 genes were substantially hypomethylated in all samples obtained from patients with chronic lymphatic leukemia. Hypomethylation of genes, particularly growth-related sequences, might be a crucial fact in the malignant transformation of human leucocytes. Its relatively simple detection from blood samples may prove clinically applicable in monitoring patients with chronic lymphatic leukemia.