A short sequence of amino acids including Lys-128 is required for the normal nuclear accumulation of wild-type and deleted forms of SV40 large T antigen. A cytoplasmic large T mutant that lacks sequences from around Lys-128 localizes to the nucleus if the missing sequence is attached to its amino terminus. The implication that the sequence element around Lys-128 acts as an autonomous signal capable of specifying nuclear location was tested directly by transferring it to the amino termini of, B-galactosidase and of pyruvate kinase, normally a cytoplasmic protein. Sequences that included the putative signal induced each of the fusion proteins to accumulate completely in the nucleus but had no discernible effect when Lys-128 was replaced by Thr. By reducing the size of the transposed sequence we conclude that Pro-Lys-Lys-Lys-Arg-Lys-Val can act as a nuclear location signal. The sequence may represent a prototype of similar sequences in other nuclear proteins.