Interstate Quarantine Regulations require that water supply systems serving planes, trains, buses, and vessels engaged in interstate commerce comply with certain minimum health and safety standards, as set forth in the Public Health Service Drinking Water Standards (1962). The EPA published 'A Guide to the Interstate Carrier Water Supply Certification Program,' in June 1971, which was revised in 1973. The Guide compiled various policies and procedures that had been developed over the years and provided a specific basis for classification depending upon the degree of compliance with the Drinking Water Standards. This publication consists of the most recent chemical data submitted by the EPA regional offices on the interstate carrier reporting forms through August 17, 1973. The data have either been furnished by the State agencies concerned with the certification program or are from analyses performed by EPA laboratories.