1 This symposium was presented at the Fifty-sixth General Meeting of the Society of American Bacteriologists at Houston, Texas, on May 3, 1956. Participants were: Martin Alexander, Cornell University, Ithaca, New York; Carl E. Georgi and W. E. Militzer, University of Nebraska, Lincoln, Nebraska; I. Millman, R. W. Darter, G. P. Youmans, and A. S. Youmans, Northwestern University Medical School, Chicago, Illinois; A. F. Brodie, C. T. Gray, and P. E. Hartman, Leonard Wood Memorial Laboratory, Harvard Medical School, Boston, Massachusetts; M. F. Utter and D. B. Keech, Western Reserve University Medical School, Cleveland, Ohio; P. M. Nossal, University of Adelaide, Adelaide, South Australia; H. P. Klein, Brandeis University, Waltham, Massachusetts; and Z. K. Booher, University of Washington, Seattle, Washington.