1. The Marine Biological Laboratory, Woods Hole, MA, U.S.A.
2. The Department of Zoology, University of California, Davis, CA, U.S.A.
3. The Departments of Laboratory Medicine and Medicine, University of California School of Medicine, San Francisco, CA, U.S.A.
4. The Department of Microbiology and Immunology, Downstate Medical Center, State University of New York, Brooklyn, NY, U.S.A.