Study of 14 human infiltrating breast carcinomas revealed new features that shed light on the pathogenesis of tumor stroma formation and on host immunologic defense mechanisms. Fibrin deposits were observed in the stroma of all tumors, particularly at their growing edge. Fibrin may have contributed both to tumor angiogenesis and, with organization, to the formation of the fibrous stroma characteristic of these and other scirrhous carcinomas. We previously proposed similar mechanisms for several animal tumors. All breast carcinomas studied elicited some degree of lymphocytic response at the tumor periphery; lymphocytes penetrated the fibrous tumor stroma poorly, did not exit in significant numbers from central tumor vessels, and, even when greatly outnumbering tumor cells locally, appeared relatively ineffective at tumor cell killing. Microvascular endothelial cell damage was frequently observed and may have been responsible for zones of tumor infarction. Similar observations have been made in skin allografts and animal tumors where rejection was effected principally by microvascular damage and subsequent tissue infarction, not by lymphocyte contact with individual epithelial target cells.