Two classes of polyomavirus large T antigen were distinguished, differing in their modes of association with the cell nucleus. A weakly associated class, the nucleoplasmic T antigen, representing 30 to 40% of the total, was solubilized when cells were lysed isotonic buffer at pH 7.2. A more tightly bound class retained in isolated nuclei, the retained T antigen, was extractable either at pH 9.0 or in 2 M NaCl. The retained T antigen contained an additional mole of phosphate, 4 mol of PO4 per mol of T antigen, compared with the nucleoplasmic T antigen (3 mol of PO4 per mol of T antigen). Limit digestion with staphylococcal V8 protease yielded equivalent amounts of five peptides ranging in size from 7.5 to 20 kilodaltons. Additional phosphorylation within a 12-kilodalton peptide accounted for most of the difference in phosphate content between retained and nucleoplasmic T-antigen classes.