In short-term liquid cultures of leukemic blood cells from 60 untreated patients with acute lymphoblastic leukemia (ALL) or acute myeloblastic leukemia (AML) the 3H-thymidine incorporation was measured. After 24, 48 and 72 h of culture, leukemic cells from AML patients consistently showed much higher incorporation values than those from ALL patients. In addition, the cultured cells were studied with the pulse cytophotometer to measure the cell cycle phase distribution of the leukemic cell population. The percentages of cells in S, G2, and M phases in cultures from AML patients were also clearly higher than those from ALL patients. These test methods provide an additional diagnostic aid, especially for those forms of acute leukemia, which remain unclassified by conventional morphological and histochemical criteria.