Suckling rats infected by ic route with 10(3)LD50 of the XJC13 strain of JV were passively immunized with homologous hyperimmune serum (HIS). Animals treated at 2 days pi with HIS showed a significant increase in survival vs. non-treated infected controls (82% vs 5%). However, at 4 days pi, transfer failed to modify survival. By means of DEAE Sephadex A25 column chromatography, the presence of neutralizing immunoglobulin closely correlated with protective antibodies, but were not restricted to the IgG-containing fraction. Employing Sephadex G200, chromatography demonstrated the absence of neutralizing and protective activity in the high molecular weight protein fraction. Results show that the success of HIS treatment depends on early administration. Besides, it was found that fractions capable of conferring protection exhibited high neutralizing antibody titers.