Our aim is to study whether or not in vitro sensitivity of leukemic cells correlates with clinical effect; if so, in vitro testing might used for stratification of treatment. During 1995-1997 bone marrow samples from 145 Swedish children with newly diagnosed acute lymphoblastic leukemia were analysed by the automated fluorometric microculture cytotoxicity assay. Therapeutic effect was evaluated by bone marrow morphology day 15 and 29. Preliminary results indicate that marrow samples from patients with poor response to induction therapy show a higher degree of in vitro resistance to several cytotoxic drugs at diagnosis than good responders.