Fifteen bus drivers, operating diesel converted trolley buses, experienced symptoms including watery and itchy eyes, rhinorrhea, and headaches. A total of 49 buses were labeled as "problem buses" and operators refused to drive them. An investigation identified high fungal counts in some problem buses (> 70,000 colony forming units [CFU]/m3; n = 3) compared with control buses (< 220 CFU/m3; n = 4). The predominant species were Penicillium and Cladosporium (1/1). Remedial measures, including washing with a 0.17% solution of sodium hypochlorite and an ozone treatment, were not successful. Because fungal species are heat sensitive, two buses were subjected to a heat treatment of 55 degrees C for 4 hours. In one bus the fungal spores of Cladosporium appeared to be more heat sensitive than the spores of Penicillium. At this point the interior of one bus was completely renewed and another was given a formaldehyde treatment followed by heat treatments. Both strategies reduced fungal counts to 190 from > 107,000 CFU/m3 for the former and to 270 from > 71,000 CFU/m3 for the latter. Only the interior of the most heavily contaminated buses were refurbished prior to the heat treatment, which was done on all problem buses. All buses are still in active service 5 years later. The most frequent health symptoms reported by 88 exposed bus drivers were headache (36%), blocked/runny/itchy nose (26%), nausea (26%), and dry irritated throat (25%). No chronic health effects have been reported after 5 years, although some of these common fungal species are known to be opportunistic pathogens.