Systemic administration of protamine sulphate significantly decreased the intensity of angiogenesis induced in X-ray immunosuppressed (BALB/c X DBA/2W) F1 mice by either HEp-2 (human larynx carcinoma) cells or semi-syngeneic splenocytes injected intradermally. In vitro experiments have shown that protamine sulphate markedly decreases the proliferation of human endothelial cells as assessed by 3H-TdR incorporation assay. In contrast, the proliferation of HEp-2 cells was not affected, and only slight inhibition of normal human fibroblasts could be demonstrated. Heparin abolished the inhibitory effect of protamine sulphate, both in vivo and in vitro. These results suggest that the observed inhibitory effect of protamine sulphate on angiogenesis in vivo may be at least partially due to the interaction of this compound with endothelial cells.