The nu/nu mouse xenograft is the only experimental system permitting the growth of human papillomaviruses (HPV). Previous studies demonstrating inhibition of HPV-11 infection by antibodies against HPV-11 virions have used indirect markers of infection, such as graft size and histopathologic features. The presence of HPV-11 mRNAs was used as a direct marker of infection: Infectious HPV-11 was incubated with rabbit serum raised against purified HPV-11 virions or with the corresponding preimmune serum (controls) before use in the mouse xenograft model, and HPV-11 mRNAs were detected by a method using reverse transcription and amplification by polymerase chain reaction. Graft size, histopathologic features, and the presence of capsid antigen were also assessed. Six weeks after infection, 1 of 23 grafts in the test group contained HPV-11 mRNAs compared with 19 of 20 controls (P less than .001). Therefore, antibody-mediated inhibition of infection by HPV-11 leads to blockade of genomic expression and is thus consistent with active prevention of viral penetration, that is, neutralization.