A procedure is given for the coupling of fluorescein isothiocyanate to concanavalin A, a protein which specifically combines with a variety of polysaccharides, and for the subsequent isolation of the reactive conjugate. This fluorescent conjugate stains Saccharomyces cerevisiae but not Schizosaccharomyces pombe or Rhodotorula glutinis. The cell walls of the latter two organisms do not contain branched homopolymers of alpha-linked mannose. Furthermore, the staining of S. cerevisiae is competitively inhibited by either unlabeled concanavalin A or methyl-alpha-d-manno-pyranoside. On the basis of this evidence, it is concluded that the staining of S. cerevisiae results from the specific interaction of the fluorescein-concanavalin A conjugate with the alpha-mannan present in the cell wall of this yeast.