Primary amino groups may be selectively labelled in living cell cytoplasmic components by staining with the covalently binding fluorochrome reagent fluorescamine. The reaction is extremely rapid and occurs at very low reagent concentrations. Cells survive such treatment and gradually remove or metabolize the labelled substances. Nuclei and nucleoli are not labelled, while lamellar cytoplasm, which contains little actual cytoplasm, is well demarcated, thus indicating that the method is useful for studies of cell membrane components. Labelled cell membranes can be prepared for further purification after preliminary external fixation of cell membranes with a supravital polyaldehyde fixative. The use of dimethylsulfoxide as a solvent for fluorescamine allows much longer survival of the cells than the use of acetone as a solvent. In addition, the use of high pH and borate buffer was not necessary to the labelling phenomenon.