Previously, we have shown that a protein designated p36 is synthesized at a high rate during interleukin 2-driven proliferation of a cloned T lymphocyte, L2. Biosynthesis of p36 increases 1000-fold during the initial mid-G1 phase of the cell cycle and remains high while the cells proliferate. In this report, we show that p36 has the same migration pattern by two-dimensional gel electrophoresis as proliferating cell nuclear antigen (PCNA)/cyclin and that antiserum to PCNA/cyclin selectively immunoprecipitates p36. In addition, by indirect immunofluorescence, PCNA/cyclin accumulates in the nucleus of interleukin 2-stimulated L2 cells during proliferation and is not detectable prior to the initial S phase or after proliferation ceases. These data indicate that PCNA/cyclin expression is induced by interleukin 2 and that PCNA/cyclin accumulation is closely associated with T lymphocyte proliferation.