We have identified and described the characteristics of a unique tumor rejection antigen (tumor-specific transplantation antigen) obtained from the murine malignant melanoma S91. This antigen is highly restricted to the autologous melanoma and provides striking inhibition of its growth. Previously, we described common or shared tumor-specific transplantation antigens on the murine malignant melanomas B16 F10, K1735, JB/RH, and JB/MS. No cross-reactivity was obtained in this study between S91 and those four other malignant melanomas. The common tumor-specific transplantation antigen resides on a glycoprotein molecule with a molecular weight of 65,000, termed B700, that shares homology with serum albumin as determined by NH2-terminal amino acid sequencing. B700, however, purified from S91 proved to be ineffective as an immunogen.