Circulating immune complexes in syphilis

13 patients with syphilis were investigated regarding the presence of circulating immune complexes by the methods of C 1q-binding-activity and anticomplementarity. Elevated C1q-binding-activity was demonstrated in 6 of 7 patients with secondary syphilis, a significantly greater incidence than among patients with primary syphilis and neurosyphilis. Anticomplementarity was demonstrated in five of seven patients with secondary syphilis and in two patients with neurosyphilis. Anticomplementarity was found in only one of four patients with primary syphilis. The presence of immune complexes may be of importance in the aetiology of some of the lesions of secondary and perhaps tertiary syphilis.