BU12-SAPORIN was selectively cytotoxic in vitro in a dose-dependent manner for the CD19+ human common acute lymphoblastic leukaemia (cALL) cell line NALM-6 but exhibited no toxicity for the CD19- T-cell acute lymphoblastic leukaemia (T-ALL) cell line HSB-2. This evidence concerns the gene CD19 and acute lymphoblastic leukemia.