A factor present in normal rabbit sera enhanced the neutralization of the infectious complex of poliovirus with the pepsin Fab' fragment of rabbit immunoglobulin G (IgG) antibody but showed no activity for the infectious complex of the same virus with IgG antibody. The factor was associated with IgG, and its activity was inhibited by the pepsin Fab' fragment of normal rabbit IgG. The data suggest that the enhancing factor belongs to naturally occurring antiglobulin to the homologous Fab' fragment (pepsin homoreactant).