The streptococcal antigen was determined in the plasma of 26 patients with Behçet's disease, 30 patients with other uveitis (tuberculosis: 7, sarcoidosis: 6, Vogt-Koyanagi-Harada's disease: 6, toxoplasmosis: 6, trauma: 5), and 63 normal subjects with matched ages. The antigen titer in Behçet's patients, either during ocular attack or remission, was significantly higher than that in the normal control subjects (P less than 0.0001) and that of other uveitis subjects (P less than 0.001). Time-course study in the Behçet's disease cases showed that the titer was the highest during the attack and then decreased gradually as the ocular inflammation subsided. It was also found in two cases that the titer increased markedly just before the ocular attacks. There was no difference between the titer in other uveitis cases and that in the controls. By electron microscope, streptococcal L form-like structures were observed in the plasma and leukocyte layer of the blood of Behçet's patients sampled during the ocular attacks. Streptococcal infection may play a significant role in the pathogenesis of Behçet's disease.