The complement-fixation test for the diagnosis of some Rickettsial diseases is proving satisfactory through the use of antigens prepared from infected chick embryos (1, 2). In these cases, the embryos are inoculated according to the method of Cox (3); when infected, the yolk sacs are triturated in saline and after removal of gross particles by slow centrifugation, the supernatant fluid is recentrifuged at high speed. The resulting sediment is then resuspended in 0.85 per cent saline up to its original volume and constitues the complement-fixing antigen.
The commercially-prepared typhus vaccine is similarly manufactured from infected chick embryos (4). The yolk sac suspension is centrifuged twice at high speed (4–5000 rpm. for one hour) and the supernatant fluid discarded. The sediment is then resuspended in saline up to its original volume and centrifuged at 1000 rpm. for ten minutes. The supernatant fluid is then treated with formalin and phenol and dispensed for prophylactic use.