Relapsed/refractory B-precursor ALL in adult patients is an aggressive malignant disease with dismal prognosis and unmet medical need. Additional therapeutic approaches are urgently needed. Blinatumomab is a bispecific single-chain antibody construct designed to link B cells and T cells resulting in T cell activation and a cytotoxic T cell response against CD19 expressing cells. The purpose of this study is to investigate the efficacy, safety and tolerability of different doses of the bispecific T-cell engager blinatumomab in adult patients with relapsed/refractory B-precursor ALL. Patrticipants will receive up to five 4-week cycles of intravenous blinatumomab treatment.