Serum from all seropositive dogs (20/33) was pooled and diluted with a solution of 1% ovalbumin in neutral phosphate buffered saline to a concentration of 1:200. Following the method of Haines and Chelack (2), this pooled serum was
used as a source of primary antibody for avidin-biotin complex immunoenzyme staining of the paraffin-embedded tissues. Negative and omission controls were achieved by the applica- tion of normal (uninfected) dog serum prepared and applied in identical manner and by the omission of any primary antibody, respectively.