For the purpose of elucibating the staining mechanism of fluorescent NTS dye that deeply stains cancer cells, some model experiments were conducted using fluorescent NTS dye mixed with a variety of proteins to pursue the manner of staining by means of stripping method at varying pHs as well as by paper electrophoresis. In addition, staining of cells was performed with the use of Ehrlich ascites tumor cells, MH-134 ascites liver cancer cells, normal rabbit liver cells, normal rabbit leucocytes, lymph nodes and peripheral blood of gastric cancer bearing patients, and human pus, and the following results were obtained. 1. Static electrical binding seems to play a principal role in the combination of fluorescent NTS dye with various proteins. 2. Cancer cells are stained deeply with the fluorescent NTS dye at higher pH range as compared with normal cells. 3. From these findings it has been clarified that, although fluorescent NTS dye is a suitable material for staining cancer cells, as young cells with abundant basophilic proteins possess specificity, fluorescent NTS dye is not necessarily so satisfactory a material with respect to its specificity to cancer cells.