A small fraction of the 94,000-molecular-weight multifunctional large T-antigen of simian virus 40 was associated with the nuclear protein matrix derived from simian virus 40-transformed mouse cells. The interaction between this fraction of T-antigen and the matrix was largely or entirely independent of nuclear DNA. Similar amounts of T-antigen were retained by the nuclei of transformed and revertant cell line. A 100,000-molecular-weight variant of T-antigen, which has been found to correlate specifically with anchorage-independent growth, was present in the nuclear protein matrix of a transformed cell line. A T-antigen-containing revertant selected for the reacquisition of a high serum requirement and an anchorage requirement for growth retained T-antigen in association with its matrix.