efficient synthesis of the title heterocyclicringsystem is described starting from suitable 2‐chloro‐1, 8‐naphthyridines. The synthesized 6H‐indolo[2, 3‐b][1, 8]naphthyridine derivatives were tested in vitro on 55 tumor cell lines for their anticancer properties. The presence of the acetylamino moiety at position 3 in the main ringsystem proved to be crucial for the cytostaticactivity of this class