The first objective of the experiment was to demonstrate that murine spleen cells treated with the T cell mitogen phytohemagglutinin induced neovascularization when adoptively transferred to the chorioallantoic membrane of chicken eggs. The second objective was to show that neovascularization could be induced by the supernatants from these cultures. The assay for neovascularization was based on the well established observation that angiogenesis can be induced on the chorioallantoic membrane by tumors and other substances. The supernatants were incorporated into a slow release polymer of hydroxyethylmethacrylate so as to produce a sustained release of the angiogenic material. The results showed that phytohemagglutinin activated spleen cells and their culture supernatants induced neovascularization on the chorioallantoic membrane. The significance of these observations are discussed as they relate to the hypothesis of lymphoid-induced neovascularization during tumor growth and other types of immunological inflammatory reactions.