Two adenovirus isolates from urine, Ad35p (from a bone marrow recipient) and Ad34a (from a hemorrhagic cystitis patient), were compared regarding their fiber gene organization and hemagglutinating capacity. The fiber serves as the ligand between the virus capsid and the host cell receptor. The Ad35p fiber gene encoded a 323-amino-acid protein, and the Ad34a fiber gene a 325 amino acid protein. The two fibers manifested 62.4% overall amino acid sequence homology, the differences predominantly occurring within the knob region where sequence homology was only 49.5%. The knob region of Ad34a was virtually identical to that of Ad11p which also causes hemorrhagic cystitis. Unlike all other known subgenus B adenoviruses, in the Ad35p fiber an asparagine constituted the C-terminus. Although both Ad34a and Ad35p viruses can hemagglutinate monkey erythrocytes, the hemagglutination inhibition test showed them to differ from each other in the epitopes expressed on the fibers.