The synthetic biopolymer NED 137 is capable of stimulating an immune response to tumor antigen. This article compares the efficacy of NED 137 to bacille Calmette-Guerin, Corynebacterium parvum, pyran, levamisole, and Freund's complete adjuvant in a rat tumor model where adjuvant treatment is administered after excision of subcutaneous tumor implants. A single intraperitoneal injection of NED 137 at 30 mg/kg body weight prolonged survival beyond 60 days with no evidence of recurrent or metastatic disease, whereas with the other adjuvants, animals survived a mean of 30 to 40 days with 100% local recurrence and a 60% to 90% incidence of pulmonary metastases. Use of NED 137 resulted in a greater lysis of tumor cells compared to other adjuvants when assessed in an in vivo 51Cr release assay. A phase I clinical study of high-risk gastrointestinal cancer patients treated with NED 137 is reported at a median survival time of 23 weeks (103 patients). This group is compared to a historical control group from the same institution. No NED 137-related toxicity has been noted to date.