Mithramycin, an antibiotic, was used in the treatment of 30 patients with disseminated germinal testicular cancer. The antitumor effect of mithramycin was evaluable in 26. Nine patients experienced an objective antitumor response, which has been complete in two. The major responses occurred in patients with embryonal carcinoma with pulmonary metastases. No significant response was observed in patients with advanced retroperitoneal or hepatic metastases from embryonal carcinoma or in the six patients with teratocarcinoma. The toxicity of mithramycin encountered in this investigation does not preclude employing this agent as a primary treatment of metastases from embryonal carcinoma of the testis.