A cytopathic factor was isolated from M. hyopneumoniae VPP11 membranes. This factor had an isoelectric point of 6.2 and was capable of inducing cytopathic effect in MRC-5 human lung fibroblast culture at a concentration of 250 ng of protein per ml. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of this cytopathic factor showed a major band with a molecular weight of approximately 54,000.