A recombinant adenovirus with deleted E1 and E3, and E4-inactivated by replacing the E4 promoter with a synthetic promoter composed of a minimal TATA box and five consensus yeast GAL4-binding site elements was developed and used to express the human tumor suppresser gene p53. The toxicity and immunogenicity of this vector and vector-mediated p53 gene expression in vivo were studied in immunocompetent C3H and C57BL/6 mice. Expression of the late viral gene product, hexon protein, was observed in C3H and C57BL/6 mice injected with E4 wild-type adenovirus constructs Adv-cmv-beta-Gal (BG), Adv-cmv-hp53 (WT), and empty E1- vector Adv-E4 (EW) 3 to 28 days after injection, but was undetectable in mice treated with E4 modified empty E1- vector Adv-GAL4 (EG) or Adv-cmv-hp53-GAL4 (G4). Expression of the p53 gene was observed in both WT- and G4-injected C3H and C57BL/6 mouse livers from days 3 to 28. Ten weeks after injection, p53 gene expression was still detected in G4-treated C57BL/6 mice at similar levels, but was not detectable in WT-treated mice. Vector-induced liver toxicity was evaluated by analyzing serum transaminases (SGOT and SGPT) activities. In all cases, SGOT and SGPT activities were markedly decreased in EG-treated C3H and C57BL/6 mice compared with those in EW-treated mice on days 3, 7 and 14 after injection. In C57BL/6 mice, the total anti-adenoviral CTL activities were two- to three-fold higher in animals treated with EW vector than in those treated with EG vector. These results suggest that inactivation of the E4 promoter efficiently diminished the viral replication and the late viral gene expression, reduced host immune response and consequently reduced toxicity and prolonged the duration of transgene expression in vivo.