The role of microenvironment interleukin 1 (IL-1) on 3-methylcholanthrene (3-MCA)–induced carcinogenesis was assessed in IL-1–deficient mice, i.e., IL-1β^−/−, IL-1α^−/−, IL-1α/β^−/− (double knockout), and mice deficient in the naturally occurring inhibitor of IL-1, the IL-1 receptor antagonist (IL-1Ra). Tumors developed in all wild-type (WT) mice, whereas in IL-1β–deficient mice, tumors developed slower and only in some of the mice. In IL-1Ra–deficient mice, tumor development was the most rapid. Tumor incidence was similar in WT and IL-1α–deficient mice. Histologic analyses revealed fibrotic structures forming a capsule surrounding droplets of the carcinogen in olive oil, resembling foreign body–like granulomas, which appeared 10 days after injection of 3-MCA and persisted until the development of local tumors. A sparse leukocyte infiltrate was found at the site of carcinogen injection in IL-1β–deficient mice, whereas in IL-1Ra–deficient mice, a dense neutrophilic infiltrate was observed. Treatment of IL-1Ra–deficient mice with recombinant IL-1Ra but not with an inhibitor of tumor necrosis factor abrogated the early leukocytic infiltrate. The late leukocyte infiltrate (day 70), which was dominated by macrophages, was also apparent in WT and IL-1α–deficient mice, but was nearly absent in IL-1β–deficient mice. Fibrosarcoma cell lines, established from 3-MCA–induced tumors from IL-1Ra–deficient mice, were more aggressive and metastatic than lines from WT mice; cell lines from IL-1–deficient mice were the least invasive. These observations show the crucial role of microenvironment-derived IL-1β, rather than IL-1α, in chemical carcinogenesis and in determining the invasive potential of malignant cells. [Cancer Res 2007;67(3):1062–71]