Two new adenovirus (AV) subgenus D serotypes, which we propose as AV candidates 48 and 49, are described. Both AV 48 and 49 were unique by serum neutralization and the pattern of restriction endonuclease fragments. By the hemagglutination inhibition assay there was a significant reciprocal cross between AV 48 and 44 and one-way crosses between AV 48 and AV 37 sera and between AV 30 and AV 49 sera. Eleven AV 48 and 5 AV 49 isolates, mainly from AIDS patients, have been identified at the Viral and Rickettsial Disease Laboratory.