1. The haemolytic complement-fixation test and the conglutinating complement-absorption test, as well as the agglutination reaction and the antiglobulin sensitization test, have been used to study a change which occurs during egg adaptation of the Christie strain of R. burneti.2. Antigens were prepared from yolk sacs of the third to the ninth passages inclusive. Irrespective of numbers of rickettsiae, antigens from the third and fourth passage failed to fix complement with homologous (human) antiserum, unless the latter was in very low dilution. From the fifth passage, however, antigens fixed complement with high dilutions of the same antiserum and thus resembled the classical Henzerling strain antigen.3. Third-passage antigen failed to fix conglutinating as well as haemolytic complement with high serum dilutions. The agglutination reaction and antiglobulin test, however, showed that third-passage antigen absorbed antibody almost as well as ninth-passage and Henzerling strain antigens.4. It was not possible to find out if the change in behaviour was due to a true antigenic variation or to non-specific hindrance of complement absorption. Heating failed to alter the behaviour of the antigens and the results of absorption tests were inconclusive.
Variation in complement-fixing activity of Rickettsia burneti during egg adaptation