Patients. Specimens of bone marrow, peripheral blood, skin fibroblasts, amniocytes and chorionic villi (CV) from 42 patients were studied. Bone marrow aspi­rates of eight patients with AML-M6, one each of persons suffering from AML-M1, AML-M4, AML-M5b and AML developing from myelodysplastic syndrome (MDS), two with AML-M2, and four with different, non malignant bone marrow diseases (leukopenia of unclear origin, auto­immune disease, neutropenia, toxic bone marrow impair­ment or thrombocytopenia and anemia) were studied. Seven bone marrow aspirates from patients with rheuma­toid arthritis were also included. All these specimens were cultivated in the presence of colcemid for 24 hours [6]. Seventy-two-hour cultures of six peripheral blood samples, three skin fibroblast, three amniocyte and one CV sample and four 24-hour cultures of different AML-cases and one 48-hour culture of AML case 38 (without permanent add­ing of colcemid) were used as controls (for further details see Table 1).