Gallium has an affinity for some human as well as animal neoplasms. Carrier-free gallium citrate Ga 67 administered intravenously is localized in a variety of malignant tumors that can then be detected on scintiscans. Nonosseous as well as skeletal lesions can be seen, including some that are not otherwise detectable. No untoward effects were observed in 41 patients studied. Scans were abnormal in 23 patients with a variety of malignant neoplasms; reticulum cell sarcoma, lymphoblastoma, and poorly differentiated adenocarcinoma gave the most consistent results. The 18 patients in whom scans were normal included three patients with plasma cell myeloma, four with lymphocytoma, and five with Hodgkin's disease involving small retroperitoneal nodes. Gallium localization is greatest in viable tumor, less in fibrotic or necrotic tumor, and diminished by irradiation and effective chemotherapy. The mechanism is unknown.