1. Departments of Cell Biology and Physiology and Pathology, University of New Mexico Health Science Center, Albuquerque, New Mexico 87131, Department of Immunology, The Scripps Research Institute, La Jolla, California 92037, Laboratory of Immunology and Vascular Biology, Department of Pathology, and Digestive Disease Center, Department of Medicine, Stanford University Medical School, Stanford, California 94305, and LeukoSite, Inc., Cambridge, Massachusetts 02142