['Adenovirus type-5 (Ad5) recombinant viruses with replacement of the 1.9 kb XbaI fragment in the early region 3 (E3) by foreign genes have been constructed with the ultimate goal of inducing immune responses to the product of the inserted gene against a variety of virus infections. The pathogenicity of these recombinants, however, has not been studied.', 'Histopathologic changes induced in cotton rat and mouse lung by E3-replacement-Ad5 recombinant or wild-type Ad (Wt-Ad) or E3-deleted mutant (Ad5-delta E3) viruses were compared. Expression of viral mRNA and replication of these viruses in cotton rat and mouse lungs, as well as in human tissue culture cells, were assayed. Expression of class I major histocompatibility complex antigens and the E3-14.7 kilodalton protein in virus-infected cells were also analyzed.', 'An Ad5 recombinant, Ad-human cytomegalovirus glycoprotein B (Ad-HCMV.gB), in which the E3 region is replaced by the full-length gB gene of HCMV and with a genome size exceeding that of Wt-Ad, induced mild histopathologic responses in cotton rat and mouse lungs, comparable with those of Wt-Ad, but less severe than those of Ad5-delta E3. Analysis indicated that neither class I major histocompatibility complex expression on the cell surface nor differential expression of the protective E3-14.7 kilodalton protein underlies the pathologic differences observed in cells infected with Ad5-delta E3 or the Ad-HCMV.gB recombinant. In the mouse lung, another Ad-E3 replacement recombinant, Ad-herpes simplex glycoprotein B (HSV.gB), containing the complete HSV.gB gene and with a genome size larger than that of Wt-Ad, also induced a very mild inflammatory response. However, two recombinants with truncated forms of the HCMV.gB (Ad-HCMV.gB.155) or HSV.gB genes (Ad-HSV.gB.147) produced more severe histopathologic changes than the Wt-Ad or the recombinants with the full complement of HCMV.gB or HSV.gB genes. Ad5 and some of the recombinants replicated in mouse and cotton rat lung, and the extent of replication was inversely proportional to genome size, both in the lung and in human tissue culture cells. Infectious virus titers were, however, higher in cotton rat than in mouse lung. In situ hybridization analysis of cotton rat and mouse lung infected with Wt-Ad, Ad5-delta E3, or Ad-HCMV.gB virus revealed expression of Ad early/late mRNA predominantly in bronchial epithelial cells.'] Are early region 3-replacement adenovirus recombinants less pathogenic in cotton rats and mice than early region 3-deleted viruses?