Bone marrow aspirates and venous blood samples from 26 subjects with acute leukemia in relapse and from 10 nonleukemic controls were incubated in vitro with tritiated thymidine and examined by autoradiography. Twenty leukemic patients were studied again during the course of chemotherapy. The number of labeled cells per 100 nonerythroid nucleated cells provided an estimate of the population, which was in the DNA-synthesis phase of the generative cycle. This ratio was used as an indicator of the

more »

... ize of the proliferative population of bone marrow. The fraction of proliferating cells was significantly smaller in the leukemic bone marrows during relapse than in the controls. In the leukemic subjects, the peripheral blood blasts showed a smaller ratio of proliferating cells than that in bone marrow, but this difference was statistically not significant. The 13 patients who eventually responded to chemotherapy showed a marked increase in the actively proliferating population in the bone marrow early in the course of therapy, whereas all those patients who failed to go into remission showed no early rise in the proliferating pool. This procedure may assist in the early detection of nonresponders to the cytocidal agents with which acute leukemia patients are being treated so that it may be possible to change the therapeutic approach before toxicity ensues.