There is a clear-cut cause-effect relationship between vascularization of a given tumor and its capacity to disseminate and metastasize. Hence, it is very important to study the contribution of the two main factors determining the generation of new blood vessels namely the tumor and the host where it grows. The influence of the tumor-host interaction on the induction of angiogenesis was studied in a tumor model-system composed by two lines of rats selected for resistance or susceptibility to a sarcoma (S-E100) and two transplantable tumors: Sarcoma E 100 and Lymphoma TACB. One line is resistant to the sarcoma but susceptible to the lymphoma while the other line behaves in the opposite way. Studying the angiogenesis generated by the s.c. implantation of each tumor in two different hosts we found that the same tumor induced different angiogenic responses according to the host in which it is growing and that the same host shows a different response according to the tumor with which it is challenged, suggesting that tumor-host interactions determine the intensity of the neovascularization process.