The antitumoral activity of an α-D-glucan from the lichen Ramalina celastri was investigated using the tumor Sarcoma 180 (S-180). Mice were inoculated with the tumor and 24 h later received a single dose of α-D-glucan (200 mg/kg). Thirty-five days after inoculation the mice were sacrificed and the tumors were examined histopathologically. Morphological analyses showed that the tumor was invasive and that it produced typical and atypical mitoses, neovascularization and an infiltration of inflammatory cells. In treated mice, inflammatory cells were more frequent and the presence of nuclear fragments suggested tumor cell death by apoptosis. The tumors of control and α-D-glucan treated mice were negative for laminin but expressed fibronectin, the intensity and distribution of which varied in the connective tissue surrounding the tumor and between tumor cells. Fibronectin expression was more intense in the connective tissue surrounding the tumor mass, in treated mice than in control mice, but in tumor cells, the expression was greater in control mice. The results indicate that α- D-glucan can inhibit tumor growth and affect host defense cell responses. The differences in fibronectin distribution between the control and α-D-glucan treated mice, suggest that this protein may play an important role in limiting the invasiveness of malignant cells.