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A variety of murine connective and epithelial tissue tumors, including the SAD/2 and FS9 fibrosarcomas, the TA3/Ha and CAD/2 mammary carcinomas and a primary methylcholanthrene-induced sarcoma, were found to contain a high proportion of cells with receptors for the Fc portion of immunoglobulin G ("Fc receptors"). Experiments were undertaken to assess whether these cells were neoplastic, or whether they represented the infiltration into the tumor of non-malignant host cells such as macrophages or lymphocytes. It was found that long-term established in vitro cell lines of the TA3/Ha SAD/2 and CAD/2 tumors were entirely negative for the Fc receptor, whereas injection of these cells led to the formation of tumors containing a high proportion of Fc receptor-bearing cells. Many of these cells were actively phagocytic as assessed by ingestion of iron filings or antibody-coated erythrocytes. Injection of Fc receptor-negative cultured tumor cells into F1 hybrids, in which host cells could be distinguished from the tumor cells by anti-H2 sera, revealed that many or all of the Fc receptor-bearing cells in the resultant tumor were of host origin. In contrast to its effect on normal spleen cells, anti-theta serum treatment also partially inhibited Fc rosettes, suggesting a T-lymphocyte origin for some of the Fc receptor-bearing cells. Since almost all cells with potential anti-tumor activity bear Fc receptors, it is suggested that an index of host cell infiltration of carcinomas and sarcomas can quickly and easily be ascertained by enumeration of Fc receptor-bearing cells.