The People's Republic of China conducted an atmospheric nuclear weapons test over the Lop Nor testing area in Southwest China at 3:00 a.m., EDT, on September 17, 1977. Based on past experience, EPA expected that radioactive fallout from this event might be measureable but not excessive in the United States. For several weeks following this event, EOA monitored for fallout by fully activating the Environmental Radiation Ambient Monitoring System (ERAMS). Fallout radionuclides on airborne particulates, in precipitation, and in cow's milk were detectable at many sampling locations throughout the United States. Maximum individual doses for all nuclides detected in air and milk following the event were calculated for six organs (bone, liver, thyroid, kidney, lung, and GI-LLI), total body, and skin. The highest individual dose was for the (131) I-milk-thyroid pathway. This thyroid dose was a factor of 4 higher than the maximum lung and bone doses and about a factor of 20 higher than the other doses. U.S. population doses of 150,200 man-rem to the lung, 127,700 man-rem to the thyroid, and 107,600 man-rem to the bone were calculated. The population doses calculated for the other organs and for total body and skin were from one-fourth to one tenth of the above doses. The calculated total body population dose was 17,200 man-rem.