In vivo- and in vitro-grown Mycoplasma hyopneumoniae organisms were inoculated onto newborn piglet tracheal organ cultures to provide a model for interaction of this organism with ciliated respiratory epithelium. Ciliostasis and loss of cilia in tracheal rings were induced by M. hyopneumoniae grown in vivo and with low-passage cultures when grown in vitro. Levels of calmodulin or dehydrogenase enzymes in tracheal ring epithelium were not altered even though ciliostasis and loss of cilia induced by M. hyopneumoniae were extensive. The capacity for inducing epithelial damage diminished with in vitro passage of the organism. Attempts to induce higher-passage cultures to attach to cilia, cause ciliostasis, or cause ciliary damage by supplementation of mycoplasmal medium with porcine lung extract failed. Epithelial damage induced by M. hyopneumoniae in tracheal rings was averted by using porcine immune serum or by separating the organisms from ciliated epithelium with a 0.1-microns-pore-size membrane. Attachment, or at least close association, of M. hyopneumoniae to ciliated epithelium appeared to be necessary to induce ciliostasis and loss of cilia in this model.