Abstract: Using Bacillus piliformis extracted from infected liver homogenate as bacterial antigen, an indirect immunofluorecent assay was established for the detection of anti-Bacillus piliformis antibody, and proved to be specific to Bacillus piliformis. By this method, it was found that the antibody positive rate of SPF rats maintained in isolators was 0 （0/28）, while that of conventional rats maintained in open system was 40.9% （34/88），indicating different infective rates of rats in different envi-ronments. The antibody positive rate of conventional mice maintained in the same open system was only 3%.As the bacterial antigen used in this method was derived from rat, whether the difference of antibody positive rate between rats and mice reflects the different antigenicities of the bacteria derived from different species remains for further study.

Key words: Indirect immunofluorecent assay, <i>Anti-Bacillus piliformis</i>, antibody, Rat