Two methods for identifying aggregates of intracytoplasmic A particles (referred to as A particles) at the light microscopic level are described in this paper. The first is an immunofluorescent method using antisera against these particles. In mammary tumors examined both in sections and in smears, specific fluorescence was found in the form of discrete granules, confined to the cytoplasmic hemisphere proximal to the lumen. Neither nuclear nor diffuse cytoplas mic fluorescence was confirmed.

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... features of these fluorescing granules seem to justify their identification with A and B particles in various clusters. It has also been shown that transplanted tumors contain more A particles at the sacrifice of B particles than do spontaneous tumors. Specifi cally fluorescing granules were also observed in the cyto plasm of smears of mouse leukemia cells that were known to carry A particles. These granules were stained red by the second method described, with fuchsin acid and méthylène blue as stains, and were referred to as inclusion bodies. Specificity of fuchsin acid staining was confirmed by a direct identification of inclusion bodies with specifically fluorescing granules in the same area of a smear. Because the staining is so simple and provides permanent prepara tions, it can be effectively used for studies on A particles in mouse leukemias.