Abstract: Methods and compositions are provided for facilitating gene therapy procedures involving the transduction of target cells with retroviral vector particles in the presence of complement containing body fluids. The administration of soluble complement inhibitor molecules to body fluids prevents the complement mediated inactivation of the retroviral vector particles, and provides a safety mechanism for such gene therapy procedures, as the action of soluble complement inhibitors is transient, and any retroviral vector particles present after the return of uninhibited complement activity will be inactivated. Preferred soluble complement inhibitors for use in the practice of the present invention include complement inhibitory anti-complement component mAbs (including complement inhibitory anti C5 antibodies).