marize the major original contributions made by this method since 1956 (older papers are reviewed only as they relate to the more recent developments); (c) to relate immunofluorescence to the classic concepts of immunology. A. Fluorescent Antibody Labeling Compounds which emit light in one wave length range when illuminated by light of a shorter wave length range are fluorescent. Most fluorescent compounds that have been studied are excited by light in the ultraviolet or near ultraviolet region and emit light in the visible range. Fluorescent dyes are called fluorochromes. The coupling of fluorochromes to antibodies forms the basis of staining with labeled antibody. Briefly, three methods of coupling fluorescent dyes to proteins have been utilized to prepare conjugates for IF staining. (As used here the term "conjugates" refers to fluorochrome-labeled proteins.) According to the original procedure of Coons, Creech, Jones, and Berliner (30) an isocyanate derivative of fluorescein is prepared by reaction of amino group of the fluor with phosgene. The isocyanate reacts with free amino groups of proteins to form a carbamido linkage: 49