Monoclonal antibodies (MAbs) prepared against the yellow fever virus (YF) vaccine strain 17D (17D YF) envelope E protein were used to investigate Fc piece involvement in antibody-mediated protection against YF encephalitis in mice 17D YF passaged either in Vero cells or in mouse brain (P-YF) to increase neurovirulence was used. To avoid uncertainty concerning antibody clearance and blood-brain barrier penetration, and to directly compare protective activity with neutralization in vitro, pre-formed antibody-virus complexes were injected intracerebrally or assayed for plaque formation in parallel. F(ab')2 fragments of an IgG2a MAb that strongly neutralized both YF strains retained molar equivalent neutralizing activity in vitro, but did not protect. However, further incubation of such F(ab')2-virus antibody complexes with rabbit IgG, but not F(ab')2 anti-mouse IgG resulted in protection. To unambiguously test for Fc piece involvement in this model we derived an IgG2a isotype switch variant from a protective IgG1 MAb-secreting hybridoma and prepared F(ab')2 fragments of the derivative. Intact and fragmented antibodies exhibited weak neutralizing activity. The variant antibody failed to protect against P-YF, but against considerably less neurovirulent 17D YF its protective capacity was 10-fold higher than that of its IgG1 parent. F(ab')2 fragments of the variant did not protect. Together, these results provide strong evidence of an in vivo protective function for the anti-virion antibody Fc piece and indicate that in vitro neutralizing activity as a predictor of antibody protective capacity is dependent on Fc piece integrity and isotype.