Plasmids currently used for nonviral gene transfer have the disadvantage of carrying a bacterial origin of replication and an antibiotic resistance gene. There is, therefore, a risk of uncontrolled dissemination of the therapeutic gene and the antibiotic resistance gene. Minicircles are new DNA delivery vehicles which do not have such elements and are consequently safer as they exhibit a high level of biological containment. They are obtained in E. coli by att site-specific recombination mediated by the phage lambda integrase. The desired eukaryotic expression cassette, bounded by the lambda attP and attB sites was cloned on a recombinant plasmid. The expression cassette was excised in vivo after thermoinduction of the integrase gene leading to the formation of two supercoiled molecules the minicircle and the starting plasmid lacking the expression cassette. In various cell lines, purified minicircles exhibited a two- to 10-fold higher luciferase reporter gene activity than the unrecombined plasmid. This could be due to either the removal of unnecessary plasmid sequences, which could affect gene expression or the smaller size of mini-circle which may confer better extracellular and intracellular bioavailability and result in improved gene delivery properties.