The present invention provides for immunoliposomes that optimizes internalization of a drug into target cells bearing a characteristic cell surface marker. The immunoliposomes comprise an Fab' domain of an antibody that specifically binds the characteristic marker, an amphipathic vesicle-forming lipid, and a polyethylene glycol derivatized lipid. The invention also provides for growth-inhibiting immunoliposomes that lack growth-inhibiting therapeutic agents and yet are capable of inhibiting the growth and proliferation of target cells.