The fluorescent antibody technique has been applied for the demonstration of mouse isoantigens at the cellular level. Specific reactions were obtained by the indirect or "sandwich" technique with a variety of living normal and neoplastic cells. Isoantigens of the H-2 system and of other systems could be demonstrated as well and appeared to be localized at the cell membrane. As far as the H-2 system was concerned, the membrane localization could be confirmed on histological sections.
Different types of non-specific staining reactions have been identified and described. Pinocytosis and cell injury led to such reactions that were morphologically distinguishable from the specific "ring" reaction and as far as pinocytosis is concerned, could be easily avoided by reducing the incubation time. In addition, a non-specific staining reaction morphologically indistinguishable from the specific "ring" reaction could be seen in a small proportion of bone marrow and lymph node cells but in no other cell type studied. The possible nature of this reaction is discussed.