This report contains a listing of environmental radiation monitoring data collected in the vicinity of Three Mile Island (TMI) following the March 28, 1979 accident. These data were collected by the EPA, NRC, DOE, HHS, the Commonwealth of Pennsylvania, or the Bethlehem Steel Corporation. The original report was printed in September 1979 and the update was released in December 1979. Volume 1 consists of the following 5 tables: Table 1-Measurements made by principal participants; Table 2-Cross-check program instituted by U.S. Environmental Protection Agency (EPA) for iodine-131 in milk. Table 3-Comparison of EPA and U.S. Nuclear Regulatory Commission (NRC) air data collected at the Three Mile Island (TMI) Observation Center; Table 4-Summary of EPA Environmental Monitoring Systems Laboratory-Las Vegas (EMSL-LV) and EPA Eastern Environmental Radiation Facility-Montgomery (EERF-Montgomery) sampling and analytical procedures; Table 5-Computer printout of environmental data collected by EPA.