MedImmune, in collaboration with GlaxoSmithKline, is developing a vaccine consisting of self-assembling, virus-like particles for the prevention of human papillomavirus (HPV) infection of the genital tract and associated cervical cancer. The vaccine candidates include the prototype vaccine, MEDI-501 (a vaccine directed against HPV-11), MEDI-503 (directed against HPV-16) and MEDI-504 (directed against HPV-18). Following phase I studies, the HPV-16 and HPV-18 components were combined into a multivalent vaccine for subsequent trials; this vaccine is known as MEDI-517. Phase II trials of MEDI-517 were initiated by January 2000 and are currently ongoing.