Mycoplasma hyopneumoniae causes enzootic pneumonia in pigs. Mucopurulent exudate, composed primarily of neutrophils and macrophages, is a common feature in airways of infected pigs during early to mid-stage infection. Secondary pneumonia due to bacterial infections with resident flora is often associated with M. hyopneumoniae infection. The effect of infection by this organism on the histochemical characteristics of cell-associated airway mucin and the morphology of epithelial cells of pigs was studied. Bronchial goblet cell sulphomucin and sialomucin were quantitated by image analysis of lung sections stained with high iron diamine/Alcian blue. Goblet cells in bronchi of infected pigs contained significantly less total mucin and sialomucin, and significantly more sulphomucin than goblet cells of age matched control pigs. Increased sulphated mucin may indicate altered glycoprotein production or hypersecretion of mucus in response to infection with M. hyopneumoniae;To determine if M. hyopneumoniae altered neutrophil function, intracellular calcium flux was examined after incubation of porcine neutrophils with or without M. hyopneumoniae. After zymosan stimulation, intracellular calcium levels were significantly increased in neutrophils incubated with M. hyopneumoniae. Enhancement of the zymosan-induced intracellular calcium flux suggested that M. hyopneumoniae may modulate calcium dependent response elements of porcine neutrophils;Mechanisms utilized by this organism to damage ciliated respiratory epithelium were examined using newborn piglet tracheal organ cultures as an in vitro model to measure loss of cilia and/or ciliary activity. The effect of in vivo and in vitro propagation of M. hyopneumoniae strain 232 on cytotoxic potential was examined. Levels of dehydrogenase enzymes or calmodulin in tracheal ring epithelium were not altered even though ciliary loss and ciliostasis induced by low passage M. hyopneumoniae strain 232 was extensive. Cytotoxicity of M. hyopneumoniae for tracheal rings was averted by porcine immune serum or by separating organisms from ciliated epithelium with a membrane. Attachment of M. hyopneumoniae to ciliated epithelium was necessary to induce ciliostasis or loss of cilia in this model;Cytotoxicity of M. hyopneumoniae for tracheal ring epithelium diminished with in vitro passage of the organism. High passage cultures of strain 232 were not induced to cause ciliostasis or loss of cilia when grown in mycoplasma medium supplemented with porcine lung preparations. Passage of M. hyopneumoniae in vitro may alter expression of genes responsible for synthesis or translocation of proteins required for cytotoxicity.