Radioactivity in the southern and eastern parts of the Darby Mountains, Seward Peninsula, Alaska, appears to be directly related to the occurrence of granite. Concentrates from placers derived from areas containing granite are more radioactive than concentrates from placers not derived from the granite and, generally, contain from 0.01 to 0.05 percent equivalent uranium. The radioactivity of these concentrates is largely due to radioactive elements in common accessory minerals in granite, such as sphene, allanite, zircon, and, locally, monazite. Locally, in the Clear Creek-Vulcan Creek rea, the headwaters of the Kwiniuk River, and on Golovnin Bay near McKinley Creek, concentrates from placers derived from granitic terrain contain as much as 0.1 percent equivalent uranium. The higher radioactivity of the concentrates from the Clear Creek area and on Golovnin Bay is due chiefly to an unidentified uranium-titanium niobate, whereas the higher radioactivity at the headwaters of the Kwiniuk River is due to thorianite.