Eleven mycoplasma strains isolated from several organs of diseased seals during the seal epidemic in the Baltic and North Sea in 1988 and the seal epidemic along the New England coast in 1979-1980, were investigated for their cytotoxic potential by inoculation into tracheal organ cultures of SPF and gnotobiotic rats. The mycoplasma strains belonged to three different species. Administration of 2.0 x 10(2) to 4.5 x 10(7) colony forming units of all of the mycoplasma strains caused an inhibition of the ciliary activity and cytopathological changes within the epithelial cells appearing as bleb-like protuberances by observation with the inverted microscope. There was, however, a distinct difference in the strength of the cytopathological changes caused by the three mycoplasma species. Semi- and ultrathin sections of tracheal rings infected with the seal mycoplasma strain 852, observed by light and electron microscopy, revealed a heavy tissue injury of the tracheal epithelium and dumbbell-shaped mycoplasmas adhering to degenerated epithelial cells.