Several group IIIa metal salts, eg, aluminum nitrate, gallium nitrate, indium nitrate, and thallium chloride, have been evaluated for in vivo toxicity in mice and rats, for cytotoxicity in tumor cells in vitro, and for activity against a broad spectrum of experimental rodent tumors. The position of these agents in the periodical table roughly parallels their toxicity, the LD50s decreasing with increasing atomic weights. This parallel also exists with regard to in vitro cytotoxicity to Walker 256 carcinosarcoma cells. Although all of the metal salts had activity against the ascites Walker 256 carcinosarcoma, they were ineffective in ascites leukemias, plasma cell tumors, or Ehrlich carcinoma. Gallium nitrate was particularly active against solid tumors transplanted subcutaneously, suppressing the growth of six of eight tumors more than 90%. Because of its demonstrated antitumor activity in rodents and its uptake and concentration by various animal and human tumors, gallium nitrate has potential usefulness in the treatment of solid tumors in man and has been entered into a phase I study at the National Cancer Institute.