This report presents the findings of a voluntary program conducted over a 10-month period during which school buses were tested for carbon monoxide (CO) levels under different climatological conditions. The objective of the test program was to determine whether or not there are any serious CO intrusion problems or indications of potential problems on a small sample of the nation's school buses. The program was not designed to gather data from a statistically significant sample size; however, 645 tests were conducted in all areas of the United States, under varying test conditions, and using test equipment with different accuracies. Test results showed, based on a recommended level of 20 parts per million (PPM), that 7.2 percent of the buses tested exceeded this level, and 5.4 percent of the buses tested had maximum CO readings above 50 PPM. Recommendations included (1) development of advisories for concerned state, local, and private school bus agencies defining necessary maintenance and inspection procedures that will reduce or eliminate the CO intrusion problem; (2) promulgation of a standard defining maximum allowable CO levels for school buses; and (3) additional CO testing under controlled conditions and with an expanded data base. (Author/MLF)