The efficiency of gene transfer into human blood monocyte-derived macrophages has been evaluated using a replication-defective adenovirus vector harboring a lac Z gene of E. coli as a reporter gene. Whereas, no beta-galactosidase activity was found in freshly infected purified monocytes, 40% to 80% of infected macrophages which derived from these monocytes showed a beta-galactosidase activity, 2 to 4 days after infection and lasted for at least 3 weeks. Moreover, beta-galactosidase activity was found in infected monocyte/macrophages 7 days after their injection into a human tumor preestablished in nude mice. These data indicate that it is possible to transfer and stably express a gene of potential therapeutical function into human monocyte-derived macrophages using an adenovirus vector.