An adherence assay has been developed utilizing suspensions of porcine tracheal cells and Mycoplasma hyopneumoniae. Location of the mycoplasmas on the ciliated tuft denoted existence of specific host cell receptors in that area. Sulfur containing molecules partially inhibited adherence by competitively blocking specific attachment sites on the mycoplasmal surface, indicating that sulfur is involved in the composition of putative host cell receptors. However, attachment was not specifically inhibited by specific IgG and IgA antibodies against M. hyopneumoniae. Attachment was sensitive to absence of salt, to low temperature, and to tetramethyl-urea, denoting existence of hydrophobic interactions. In a second study, the surface of M. hyopneumoniae has been observed to be weakly hydrophobic, as measured by partitioning in hydrocarbon, salt aggregation test, and hydrophobic interaction chromatography. Electron microscopic assessment of mycoplasma cells that had been treated with trypsin revealed that proteins were located mainly in an intermediate layer between the cell membrane and an outer layer of ferritin-labeled material. Periodate treatment of cells did not change hydrophobicity, although periodation removed the ferritin-labeled outer layer of the surface of the organism, denoting its carbohydrate composition. In other work, two different passage levels (p27 and p76) of strain 232 FA1 of M. hyopneumoniae were observed to differ in pathogenicity for swine. The lower passage strain was able to induce pneumonia in 4 of 8 pigs inoculated, whereas the higher passage strain failed to cause pneumonia in any of 8 pigs inoculated; however, the organism was recovered from lungs of 6 of the 8 pigs. No differences were observed in SDS-PAGE protein profiles of these two passage levels of strain 232 FA1. Immunoblots of the antigens separated by SDS-PAGE and probed with a limited library of monoclonal antibodies were identical. Utilizing a colony immunobinding assay, no evidence of phenotypic switching of the pathogenic strain 232 p27 was observed. No differences were detected in colony transparency of strain 232 p27. In conclusion, attachment of M. hyopneumoniae to host tissues was shown to be a multiphasic event mediated by specific and nonspecific (hydrophobic) interactions between the mycoplasmal glycocalyx and the ciliary membrane of host cells. (Abstract shortened with permission of author.)