Evaluation of virulence in Mycoplasma hyopneumoniae strains 168 and 168L. After treatment, piglets challenged with strain 168 showed symptoms of coughing, but no clinical signs of pneumonia were observed in the 168L or control groups. All pigs were alive during the entire experimental period. Lung lesion scores were subjected to statistical analysis after slaughter. The main lung lesion of the strain 168 challenged group is the pulmonary consolidation in the cranial and middle lobes of the lungs, and the lesions have obvious boundaries with the non-lesioned areas. Significant differences were observed between groups challenged with M. hyopneumoniae strain 168 and 168L (p < 0.05).