Fluoride given as a high single dose to young rats with developing molars has earlier been shown to cause subameloblastic cysts with disorganized ameloblasts in the cystic wall and an irregular mineralization pattern of the underlying enamel. In the present study immunohistochemistry has been employed to determine if an increased permeability of the enamel organ occurred at the areas of cell disturbances. For this purpose 5-day-old rats were injected with 60 mg sodium fluoride per kg body weight. They were decapitated after 24 h, the maxilla prepared histologically and paraffin sections incubated for the demonstration of IgG according to the avidin-biotin-peroxidase complex method. Staining for IgG was present in the cystic lumina and in areas of disorganized ameloblasts. No reaction was observed in areas of unaffected ameloblasts. It was suggested that the fluoride-induced cell injury increased the permeability of the ameloblastic cell layer. Diffusion of IgG and most likely other substances as well, through the ameloblastic layer may have contributed to cyst formation and to the irregular mineral deposits that have been found in the ameloblastic layer and at the enamel surface.