It is believed that preformed antibodies are responsible for blood transfusion reactions and transplant rejections. In order to remove a tumor, the tissue must be rejected. On the basis of transfusion reaction and transplantation immunology, we hypothesized that allogeneic serum can inhibit tumor growth when injected intra-tumor. Initially, an in vitro cytotoxicity test was conducted using the C57BL/6 serum (intact or decomplemented) in combination with the BALB/c-originating CT26 cell line. The CT26 cell line was used to establish a mouse model of colon cancer. When the tumor was palpable, C57BL/6 serum was injected intra-tumor. In addition to tumor size, hypoxia, metastatic capacity, angiogenesis, and metabolic and inflammatory status, we evaluated matrix metalloproteinase-2 (MMP)-2 and 9, vascular endothelial growth factor (VEGF)-A, Cluster of Designation (CD) 31, CD38 and interleukin (IL)-10. An in vitro experiment showed that heat-inactivated C57BL/6 serum had significantly lower cytotoxic effects on BALB/c-derived CT26 cells than intact C57BL/6 serum or BALB/c serum. In vivo experiments revealed that tumor size, HIF-1α, , MMP-2, and MMP-9 levels were significantly lower in the experimental group than in the control group. . In contrast, to control animals, allogeneic serum treatment led to marked reductions in CD31, VEGF-1, CD38, and IL-10 levels. A new approach to serum or plasma therapy and allogeneic vaccines for cancer is the intra-tumor injection of allogeneic serum. In light of the ease and availability of allogeneic immunotherapies, allogeneic serum and plasma therapy could potentially be used as an alternative monotherapy or in combination with other therapies.