The WKA/Mk rat tumors induced by Friend virus complex, Rauscher virus complex, and their associated lymphatic leukemia viruses were investigated for their antigenic relationhips with transplantation experiments and cytotoxicity tests. It was found that Friend lymphatic leukemia virus-induced tumors lacked part of the tumor-associated transplantation antigens (TATA's) on Friend virus complex-induced tumors, and the former did not express the type-specific (Friend) TATA for the latter not shared by Rauscher virus complex-induced tumors, which was previously reported by the authors. In contrast, the antigenic differences between TATA's of Rauscher virus complex-induced tumors and those of Rauscher lymphatic leukemia virus-induced tumors were not clearly demonstrated. Furthermore, these studies indicated that Rauscher lymphatic leukemia virus-induced tumors had a weak type-specific TATA not shared by the tumors induced by Friend lymphatic leukemia virus. These results of transplantation studies were also serologically supported by cytotoxicity tests.