B a c k g r o u n d. Mature B-cell acute lymphoblastic leukemia (B-ALL) is a rare type of leukemia in children and counts for 1-2% of all leukemia cases. Pediatric BALL is treated according to protocols for non-Hodgkin lymphomas. O b j e c t i v e. The aim of the study was to analyze the in vitro drug resistance in children with BALL compared to ALL with other phenotypes. M a t e r i a l a n d m e t h o d s. A total number of 15 children with BALL (6 girls and 9 boys, median age 8 years, range 1.9-15 years) were included into the analysis. The in vitro drug resistance tests on leukemic cells were performed by means of the MTT assay. The results of BALL patients were compared to those obtained in patients with pre