After observing the presence of numerous stromal myofibroblasts in scirrhous mammary carcinomas, a series of invasive and metastatic carcinomas from diverse sites was examined by electron microscopy to determine whether myofibroblasts might also be present in their stroma. Myofibroblasts were identified in each instance and were most abundant in neoplasms which were hard, sclerotic, and retracted. This finding suggests that myofibroblasts represent a component of the stromal reaction to many carcinomas and contribute to the desmoplasia and retraction which characterize many of these neoplasms. The host commands several responses to neoplasia. As a result of the expression of tumor-associated antigens, the immune system contributes lymphocytes, macrophages, and antibodies, a reflection of immunologic surveillance against neoplasia. In contrast to experimental systems tumor neoantigens are poorly expressed or even lacking in many human neoplasms; thus, the immune system may be weakly stimulated or not activated at all. Tumor neovascularization induced by a tumor-angiogenesis factor represents a second host response, possibly deleterious, for it may facilitate tumor dissemination. The stromal myofibroblast reaction to many invasive and metastatic carcinomas may constitute a third, albeit more primitive response. The density of collagen produced and contractile state of such tissue may signify an attempt by the host stroma to contain the neoplasm and impede vascular invasion. If so, myofibroblast induction may complement immune surveillance or constitute a separate mechanism of response to invasive neoplasia in man.