Lucifer yellow CH, a fluorescent hydrazide, reacted with cell surface glycoconjugates on murine thymocytes oxidized with NaIO4 or galactose oxidase. Surface labeling was monitored by fluorescence microscopy, spectrophotometry and by flow microfluorometry. No labeling was detected in unoxidized cells and fluorescence was reduced by neuraminidase and by trypsin. Lucifer yellow CH derivatives of gangliosides also were prepared and incorporated into the surface membranes of thymocytes.