An indirect immunofluorescence test (IIF) using guinea pig lip as a substrate was performed with sera from 10 patients with Behçet's disease and the results were compared to IIF with sera of 16 patients with uveitis of Turkish origin, 62 patients with non- Behçet's uveitis, 9 patients with stomatitis aphtosa and 34 healthy controls. Antibodies to guinea pig lip cytoplasmic antigens were present in 70% of the Behçet patients, in 19% of uveitis patients of Turkish origin (P less than 0.05), in 12% of the non Behçet uveitis patients (P less than 0.001), in 11% of the stomatitis aphtosa patients (P less than 0.05) and in 9% of the healthy controls (P less than 0.001). A positive Behçet serum preincubated with the supernatant of guinea pig lip sections incubated in PBS gave an inhibition on the IIF test, indicating that the antigen involved in a soluble cytoplasmic substance. No significant increase in the presence of antibodies against iris, cornea or retina (mouse eye) were detected in the Behçet sera. Of interest was the finding of a positive reaction in 6 out of 10 patients in the episcleral region of the mouse eye. This positive reaction resembles immune complex like material. Our findings suggest that using the external surface of the guinea pig lip as a substrate is useful in selecting out those patients with Behçet's disease from uveitis patients with an unknown etiology and from patients with aphtous stomatitis.