Although in some cases superantigens (SAGs) have been shown to bind directly to T cell receptor (TCR) in the absence of MHC molecules, the precise role of MHC class II in SAG presentation to T cells is not thoroughly understood. In particular, it is still not known whether MHC class II is a mere transporter of mouse mammary tumor virus (Mtv) SAG to the cell surface or an essential component complexed with SAGs for TCR triggering. In this study, we found that MHC class II negative B cell line transfected with CD72/Mtv7 sag chimeric gene could express the Mtv7 SAG on the cell surface. The murine B cell line M12.4.1 and its MHC class II negative mutant, M12C3 are transfected with CD72/Mtv7 sag chimeric gene. Although both transfectants expressed Mtv7 SAG on their cell surface, M12.4.1 but not M12C3 activated Mtv7 SAG responding T cell hybridomas. The results argue that the mere presence of Mtv7 SAG on the cell surface does not effectively transmit the signal to TCR. As MHC class II-positive cells transfected with CD72/Mtv7 sag gene caused T cell activation, the cytoplasmic/transmembrane portion of Mtv7 SAG is not essential for T cell activation. In order to examine the importance of the membrane proximal region of Mtv7 SAG in T cell activation, we constructed chimeric genes between the encoding cytoplasmic/transmembrane portion of CD72 and N-truncated extracellular region of Mtv7 sag (CD72/ATG3, CD72/ATG5). Despite the expression of Mtv7 SAG on the cell surface, cells transfected with CD72/ATG3 or CD72/ATG5 genes were unable to stimulate Mtv7 SAG responding T cell hybridomas. The results indicate that 54 extracellular amino acids (the difference between CD72/Mtv 7 SAG and CD72/ATG3) located proximal to the membrane may be important for Mtv7 SAG function.