Following interaction of the random polymer (Glu60,Phe40)n (GPhe) with antigen-presenting accessory cells (APC), unusual costimulatory activities were noted in several murine T cell systems. When GPhe, in contrast with other random copolymers (GT,GL), was added during "inhibition" and T cell "repertoire" studies as a (negative) control to GLA-reactive nonclonal T cell lines of haplotypes H-2d (DCL-2) or H-2bm12, augmentation of T cell proliferation ([3H]thymidine incorporation ([3HT]) to homologous antigen was observed. Augmentation by GPhe was also observed in the response of a GLPhe-reactive (H-2s X H-2d)F1 T cell line and the allogeneic response of the clonal T cell line D10.G4.1. This augmentation was critically dependent on the concentration of adherent accessory cells. Although the mechanism of action of GPhe remains, as yet, undefined, the GPhe-mediated enhancement of DCL-2 (a TH2, H-2d anti-GLA, T cell line) proliferation was not dependent upon the production of either IL-1 or IL-6 by accessory cells. In addition, enhanced DCL-2 proliferation was not accompanied by a significant increase in detectable IL-4 release.