The sites of virus accumulation and synthesis in plant cells have been studied by immunofluorescence techniques (1, 2). The usefulness of this approach is limited by the relatively large amounts of fluorescent antibody required for visible fluorescence. It seemed possible that a much greater sensitivity could be obtained with tritium-labeled antiserum and autoradiographic techniques. Lysozyme and ribonuclease have both been labeled by exposure to tritium gas and found to be enzymatically active (3, 4). Thus it appeared reasonable that tritium-labeled antibodies could be prepared in this way. Anti-rabbit goat globulin (Microbiological Associates, Bethesda, Md.) was used since high titer preparations are readily available. The globulin was frozen-dried and the labeling was carried out by New England Nuclear Corporation according to the method of Wilzbach (5). For a preliminary experiment, a sample of goat globulin was exposed to tritium gas at 0°C for 10 days.