It has been found previously that irradiated, IL-2 gene-modified plasmacytoma (X63-m-IL-2) vaccines are more efficient in the therapy of the parental (X63-Ag8.653) plasmacytoma than live plasmacytoma vaccines. In this communication, we have demonstrated that irradiation of murine IL-2-producing plasmacytoma vaccines resulted in upregulation of CD80 molecule expression and IL-2 production. The expression of MHC class I antigens was not altered. The upregulation of the CD80 membrane molecule expression in X63-m-IL-2 cells was higher after irradiation with 150 Gy than after irradiation with 50 Gy. Comparable upregulation of the CD80 molecule expression has also been demonstrated after irradiation of the parental murine X63-Ag8.653 plasmacytoma cells. The results indicate that upregulation of the CD80 molecule expression and enhanced IL-2 production in irradiated X63-m-IL-2 cells was responsible for the higher therapeutic effectiveness of the irradiated plasmacytoma vaccine.