Three fibrosarcomas of different immunogenicities were tested for their ability to form spontaneous and experimentally induced metastases in normal, sham-suppressed, immunosuppressed, and immunologically restored syngeneic mice. Immunosuppression, achieved by adult thymectomy and sublethal X-irradiation (450 R), affected experimental metastasis of the three tumors in different ways. Upon i.v. injection, the highly immunogenic fibrosarcoma formed more pulmonary tumor colonies in immunosuppressed mice than in normal, sham-suppressed (sham thymectomy and 450 R), or immunologically reconstituted animals (thymectomy, X-irradiation, plus 10(7) normal syngeneic lymphocytes given i.v.). A fibrosarcoma of intermediate immunogenicity also formed more pulmonary metastases in immunosuppressed recipients, but this increase could not be reversed by reconstitution with 10(7) lymphocytes. In contrast, the least immunogenic tumor formed fewer pulmonary tumor colonies in immunosuppressed mice than in normal, sham-suppressed, or immunologically reconstituted mice. We conclude that the role of the immune system in experimental cancer metastasis varies for different tumors and that tumor immunogenicity is an important factor in the relationship between host immunity and tumor dissemination.