After anesthesia (intramuscular injection of 0.11-0.15 ml of a mixture of ketamine, 83.3 mg/ml, Parke-Davis, Morris Plains, NJ, and xylazine, 3.3 mg/ ml, The Butler, Columbus, OH), rats were inoculated intranasally with 100 ll aliquots of M. pulmonis medium or sterile culture medium into each nostril daily, on three consecutive days [13] .