It has been established that at least some Vβ17+ T cells interact with an endogenous superantigen encoded by the murine retrovirus, Mtv-9. To analyze the role of major histocompatibility complex (MHC) dass II molecules in presenting the Mtv-9 encoded superantigen, vSAG-9 to Vβ17+ hybridomas, a panel of nine hybridomas was tested for their ability to respond to A20/2J (H-2d) and LBK (H-2a) cells which had been transfected with the vSAG-9 gene. Whereas some of the hybridomas recognized vSAG-9 exclusively in the context of H-2a, other hybridomas recognized vSAG-9 exclusively in the context of H-2d or in the context of both H-2d and H-2a. These results suggest that: (a) the class II MHC molecule plays a direct role in the recognition of retroviral superantigen by T cells, rather than serving simply as a platform for presentation; and, (b) it is likely that components of the TCR other than Vβ are involved in the vSAG- 9/TCR/class II interaction. © 1992, Rockefeller University Press., All rights reserved.