The discovery of specifically acquired immunity which followed the major contributions of Louis Pasteur completely over-shadowed the first studies of the host's natural resistance. Later, the exquisite sensitivity and precision of antigen-antibody reactions made the study of immunochemistry much more attractive than the rather primitive and ambiguous field of non-specific immunity. Neverthe less, during the last three decades, a considerable body of informa tion was developed and also means by which natural resistance could be enhanced or depressed by exogenous agents such as...