We have constructed an E1-defective adenovirus (Ad) vector designated AdCAG-Cre containing the Cre recombinase gene derived from bacteriophage P1 under control of the cytomegalovirus immediate early enhancer-chicken beta-actin hybrid (CAG) promoter. We examined the Cre-loxP-based recombination by this Ad vector in C2C12 cells bearing a reporter gene construct CAG-CAT-Z, which directs expression of the E. coli lacZ gene upon Cre-mediated excision of the CAT gene located between the CAG promoter and the lacZ gene. Nearly 100% of these cells were shown to start to produce beta-galactosidase after infection with the AdCAG-Cre vector at MOI 100. On the basis of this result, we discuss the possible use of the AdCAG-Cre vector to manipulate the gene expression in mammalian cells.