Human T-cell leukemia virus type I is associated with human lymphoid malignancies. The p40xI protein encoded by the x gene of this virus is believed to play some role in virally mediated transformation. This gene is known to encode a transcriptional trans activator which previous studies have shown to be a nuclear protein. Further characterization of the intracellular kinetics of this protein showed that it migrated into the nucleus very soon after synthesis. Within the nucleus, p40xI was distributed almost equally between the nucleoplasm and the nuclear matrix. Given the proposed role of the nuclear matrix in RNA transcription, the association of p40xI with the matrix places it in an appropriate cellular compartment to exercise an effect on transcription.