Further studies on an improved haemagglutination inhibition test with higher sensitivity for rabies virus antibody.

The efficiency of the removal of non-specific inhibitors of rabies virus haemagglutinin by treatment with colloidal silicic acid, which was proposed in an earlier study, was examined in a number of test samples. The non-specific inhibitors were removed in 289 out of 296 normal human sera (97.6%) by this treatment to a level that was undetectable at the 1:4 starting dilution in the haemagglutination inhibition test. Antigenic differences among three strains of rabies virus were detected in the haemagglutination inhibition test and the antibody titres to the homologous antigens were apparently higher than those to heterologous antigens.