Abstract
In a previous communication the authors drew attention to the occurrence of complement-fixation by normal serum along with various non-antigenic substances, and recorded the results of a study of this phenomenon with particular reference to the serum principle or principles concerned in the reaction (Mackie and Finkelstein, 1928). Though no conclusive interpretation of the results could be offered, a close analogy was established between the reacting agent of the serum and a natural antibody, e.g. a natural haemolysin, and the observations appeared to be of significance in relation to natural immunity phenomena.