Abstract: A composition for immunohistochemical staining which contains a diagnostic marker comprising an antibody bound with a fluorescent functional group, together with a substance selected from the group consisting of a glycerophospholipid, a fatty acid, and a surfactant consisting of a saccharide derivative. The composition has excellent fluorescence intensity and does not cause problems of damaging living tissues and DNAs due to irradiation by ultraviolet light, and hence is useful for in vivo immunohistochemical staining. For example, it enables efficient and safe quasi-internal early diagnosis of malignant neoplasia of epithelial tissues such as esophagus cancer, stomach cancer, and large bowel cancer by using infrared ray endoscope and other, or identification and diagnosis of lesions during surgical operation.

Abstract: A diagnostic marker containing (a) a detection system such as an antibody and (b) a fluorescent functional group that is bound to the detection system and represented by the following formula: ##STR1## wherein R.sup.1 and R.sup.2 independently represent hydrogen atom, an alkyl group or the like; R.sup.3 represents an alkyl group, a sulfonic acid-alkyl group or the like; X.sup.- represents an anion species, if required; Y represents a C.sub.1 -C.sub.10 alkylene group or a C.sub.1 -C.sub.10 alkylene group containing one or more atoms selected from the group consisting of oxygen atom, nitrogen atom and sulfur atom. The diagnostic marker emits fluorescence having a wavelength of 780 nm or more when irradiated with near or far infrared rays, and thus useful for infrared endoscopic diagnosis or identification of a focus in surgical operation.