In a previous study, we found 15% L-forms of bacteria (predominately coagulase negative staphylococci (CNS)) in ears which gave negative cultures by conventional methods. In this study, we used an animal model to test whether CNS and its L-forms can be pathogenic and whether L-forms have a crucial role in the tendency to secretory otitis media (SOM). We inoculated the tympanic bullas of guinea pigs, in 2 groups, with CNS and its L-forms (revertant forms). We observed that both CNS and its L-forms had the capability of causing infection. However. it was milder for the L-forms than CNS. We clearly noticed that on day 30 60% of the ears inoculated with L-forms had effusion and/or retraction of the tympanic membrane. These ears were histopathologically characterized by hypertrophied pseudostratified epithelium or stratified squamous epithelial metaplasia. The ears inoculated with the original form of CNS had only 16.66% effusion. On day 60 we observed similar findings. Thus, it might be proposed that L-forms could be responsible for chronic irritation to middle ear mucosa leading to SOM.