Retrovirus-derived vectors are currently the preferred vectors used for human gene therapy protocols. Serious safety concerns persist, however, which are specifically related to the formation of a replication-competent virus, and no synthesis method currently employed precludes its formation with certainty. For many cell types, a low transduction efficiency results in insufficient therapeutic benefit. We describe the development of a molecular conjugate system, which permits transient chemical modification of a retrovirus with polylysine. This modification not only introduces additional safety features over standard unmodified retrovirus vectors, but also provides enhanced transduction efficiency.