The precautionary evacuation of a major Chicago office building was necessitated on September 28, 1983, by an electrical fire in a transformer vault, which resulted in the loss of 15 gallons of transformer oil composed of 65 percent of the polychlorinated- biphenyl (PCB) Aroclor-1260 (11096825) and 35 percent trichlorobenzene (12002481). Environmental sampling showed that PCB contamination was limited to the vault (20,900 micrograms/square foot), the adjacent fan room (22,000 to 39,100 micrograms/gram on the filters), the ventilation shafts from the vault itself (16,800 and 14,300 micrograms/square foot) and the exterior surface of the one story plaza building (8,900 micrograms/square foot). Air levels of PCBs in the vault 4 hours after the fire was 58 micrograms/cubic meter. Analysis of soot from a surface wipe sample taken from the ceiling of the transformer vault showed 114 micrograms/square foot of total polychlorinated dibenzofurans (PCDF) and polychlorinated dibenzo-p-dioxins (PCDD). A surface wipe sample from the wall of the vault contained 20,900 micrograms/square foot of PCBs. In all four exposed individuals, serum PCB levels were less than the detection limit of 10 parts per billion. No PCDFs or PCDDs were detected.