The growth and metastasis of transplanted Lewis lung carcinoma in C57BL/6J mice were inhibited by an ip injection of whole serum from Carcharhinus plumbeus, the sandbar shark. Tumors failed to develop in 69% of the animals inoculated with shark serum on days 0, 3, and 6 after tumor transplantation. Histologic examination of the tumor site at days 3 and 6 showed that tumor cells were pyknotic, and evidence of lysis of tumor cells and minor leukocytic infiltration existed. Tumor cells were not in tissue sections from day 15, and these animals still had no symptoms at day 216. The mean tumor volume of the remaining 31% of the treated animals was less than that of controls; they had a prolonged mean survival time, but ultimately they died from metastases, as did the controls. Urea- and hemoglobin-treated animals and those pretreated or intralesionally treated with shark serum were similar to the controls in both tumor kinetics and survival times.