An in vivo model of tumor-induced angiogenesis was used to monitor two known inhibitors of angiogenesis, protamine sulfate and the steroid tetrahydro S. Tumor cells entrapped in alginate beads were injected s.c. into mice. Blood vessel induction was measured by two quantitative methods: measurement of hemoglobin at the alginate pellet site, and pooling of radiolabeled RBC to the alginate pellet site. The two methods gave parallel results. Tetrahydro S with or without heparin inhibited blood vessel growth by 50%, and protamine sulfate inhibited blood vessel growth by 85%. These results were supported by gross morphology and histological analysis of the alginate pellet site.