Cytological and cytochemical studies of green monkey kidney cells infected with SV40 virus indicated that the type of lesion produced was influenced by the multiplicity of infection and that the lesions appeared later and progressed more slowly when the inoculum was diluted. The earliest change consisted of enlargement of ribonucleoprotein-containing spherules in the nucleolus (nucleolini). This was followed by rarefaction, with or without condensation, of the chromatin and the appearance of one or more homogeneous masses of inclusion material containing DNA, RNA, and non-histone protein which eventually filled the nucleus. In some instances the chromatin appeared to be directly transformed into inclusion material. In the later stages of infection, the ribonucleoprotein of the nucleolini was no longer stainable and material resembling the nucleoprotein of the intranuclear inclusions was found in the nucleolar vacuoles and in the cytoplasm. The nucleic acids in the inclusions were stained by toluidine blue, toluidine blue-molybdate, the Feulgen stain, and by methyl green. The stainable material was extractable by nuclease digestion or by hot trichloroacetic acid. Green or yellowish green staining by acridine orange was apparently due to binding of dye by protein and not by nucleic acids since the staining reaction was not reduced by extraction of nucleic acids by hot trichloroacetic acid. Extraction with pepsin in combination with ribonuclease or deoxyribonuclease removed practically all the inclusions from the cells; consequently they could not be stained with acridine orange. The cytochemical studies suggest that the use of pepsin together with nuclease is not a meaningful technique.