The effects of M. ovipneumoniae and M. arginini on tracheal organ cultures prepared from a neonatal kid and a foetal lamb were studied. Both organisms were isolated from the cultures throughout the 14 days of observation. M. ovipneumoniae produced ciliostasis and loss of cilia, confirmed by scanning electron microscope (SEM), after 4 days. These effects were sudden and profound in lamb explants, and gradual and less pronounced in kid explants. Clusters of organisms attached to epithelial surfaces and in association with cilia were visible by SEM. M. arginini also induced ciliostasis and cilia loss in both kid and lamb explants, but onset was more rapid, at 2 days, and there was evident recovery after day 6, with apparent regeneration of cilia. No clearly recognizable mycoplasmas were observed by SEM in M. arginini-infected explants.