Eight dogs and five cats with histopathologically confirmed fibrosarcomas were treated with Acemannan Immunostimulanta in combination with surgery and radiation therapy. These animals had recurring disease that had failed previous treatment, a poor prognosis for survival, or both. Following four to seven weekly acemannan treatments, tumor shrinkage occurred in four (greater than 50%; n = 2) of 12 animals, with tumors accessible to measurement. A notable increase in necrosis and inflammation was observed. Complete surgical excision was performed on all animals between the fourth and seventh week following initiation of acemannan therapy. Radiation therapy was instituted immediately after surgery. Acemannan treatments were continued monthly for one year. Seven of the 13 animals remain alive and tumor-free (range, 440+ to 603+ days) with a median survival time of 372 days. The data suggests that Acemannan Immunostimulant may be an effective adjunct to surgery and radiation therapy in the treatment of canine and feline fibrosarcomas.