To demonstrate the exact localization of immunoglobulin and fibrin deposits in skin lesions of lichen planus, we employed the peroxidase-antiperoxidase technique, both light and electron microscopically. IgM is shown to be deposited preferentially in and on the cytoid bodies all over their cut surface in a typical stippled pattern, partly obscuring the characteristic fibrillar substructure. All other structures such as the basal lamina are devoid of IgM deposits. Fibrin (F) is also present in cytoid bodies but in a lesser amount; it is characteristically deposited on and along the basal lamina and its reduplications in the uppermost strata of the dermis. Our results indicate that IgM deposition in cytoid bodies occurs at a very early stage during the formation of these structures and that fibrinogen exhibits a specific affinity not only to cytoid bodies but also to basal lamina material.