Recombinant retrovirus vectors are widely used for gene transfer studies. The recent development of a pseudotyped Moloney murine leukemia virus vector that contains the G envelope protein from the vesicular stomatitis virus allows for efficient concentration of vector and offers hope for potential use of these vectors for gene expression in vivo. A standard amphotropic vector expressing a serum marker protein, human alpha 1-antitrypsin, was infused into regenerating mouse liver and was 10-fold more efficient at achieving stable gene expression than was an equivalent pseudotyped vector. Discrepant results were obtained with cultured hepatocytes infected with an Escherichia coli beta-galactosidase-producing pseudotype and amphotropic vector. High rates of beta-galactosidase-positive cells were detected with the vesicular stomatitis virus G glycoprotein vector under culture conditions known to be relatively nonpermissive for retrovirus-mediated gene transfer. Subsequent studies demonstrated that beta-galactosidase protein was concentrated and copurified during pseudotype vector preparation, resulting in high rates of protein transfer rather than stable gene transfer, a process referred to as pseudotransduction. The cotransfer of protein with concentrated pseudotyped retroviruses indicates that caution must be used when interpreting gene transduction efficiencies in gene therapy experiments.