This paper presents improvements to a method for detecting antigens in tissue cells using fluorescent antibody staining. The method, originally described in 1942, involves conjugating antibody with fluorescein isocyanate to create a specific histochemical stain. The authors detail the synthesis of fluorescein isocyanate, optimizing the conjugation conditions, and methods for preparing tissue sections and staining techniques. They also discuss the choice of isomer, non-specific staining, and controls to ensure immunological specificity. The method has been successfully applied to various antigens, including pneumococcal polysaccharide, rickettsiae, and mumps virus. The authors conclude by highlighting the advantages of fluorescence over other labeling materials and the limitations of the method, particularly in examining tissue suspensions and biological fluids.This paper presents improvements to a method for detecting antigens in tissue cells using fluorescent antibody staining. The method, originally described in 1942, involves conjugating antibody with fluorescein isocyanate to create a specific histochemical stain. The authors detail the synthesis of fluorescein isocyanate, optimizing the conjugation conditions, and methods for preparing tissue sections and staining techniques. They also discuss the choice of isomer, non-specific staining, and controls to ensure immunological specificity. The method has been successfully applied to various antigens, including pneumococcal polysaccharide, rickettsiae, and mumps virus. The authors conclude by highlighting the advantages of fluorescence over other labeling materials and the limitations of the method, particularly in examining tissue suspensions and biological fluids.