A new in vivo system of monitoring tumor cell induced blood vessel growth using a sodium alginate entrapment process was developed. The alginate polymer of guluronic and mannuronic acids surrounds and sequesters cells from direct contact with their immediate environment, but permits diffusible angiogenic factors to pass through to induce neovascularization in the host. The alginate beads containing tumor cells were injected subcutaneously into animals and coalesced as a point source. The tumor cells are protected from direct contact with the host's immune system, so that various tumor types may be evaluated for their angiogenic potential across histocompatibility or species barriers. C57BI/6, BALB/c and nude mice as well as squirrel monkeys were used as host animals. This provided tumor cell testing in a syngeneic, allogeneic, or xenogeneic system. We found that alginate-Lewis lung carcinoma cells were potent inducers of blood vessel growth. As few as 100 alginate-Lewis lung carcinoma cells were needed to induce macroscopically visible blood vessels by 3 days. Dose-response experiments with alginate-Lewis lung carcinoma cells showed a greater level of blood vessel induction as cell numbers increased. Neovascularization was monitored qualitatively by macroscopic photography and microscopic histologic evaluation. Also, neovascularization was monitored quantitatively by measuring the level of hemoglobin at the injection site of alginate or by measuring the amount of radioactive red blood cells pooled at the injection site of the alginate beads. Both the measured levels of hemoglobin and radiolabeled red blood cells increased at the alginate site with each log increase of tumor cells delivered, which paralleled our findings at the macroscopic and microscopic level. This in vivo angiogenesis model was relatively simple and the procedures technically easy to perform. Most importantly, this model allowed both a qualitative and quantitative assessment of tumor-induced blood vessel growth.