Polychlorinated-biphenyl (1336363) (PCB) exposure during electrical transformer maintenance operations was assessed. Breathing zone air samples were collected in vaults which housed transformers. Bulk samples of Askarel, a mixture of chlorinated-benzenes and PCB, from each transfer location were obtained and analyzed for type of PCB, contaminants, and association solvents. Premaintenance PCB levels of air inside the vaults were all below NIOSH recommended exposure criteria of 1 microgram per cubic meter, ranging from 0.1 to 0.9 micrograms per cubic meter. In areas normally travelled by nonmaintenance personnel, 55 percent of the samples were nondetectable at the 0.1 microgram concentration, the rest ranged from 0.2 to 0.6 micrograms per cubic meter. Fifty eight percent of all postmaintenance PCB samples were below the limit of detection in the vault area, ranging from 0.1 to 1.9 micrograms per cubic meter, and 80 percent of those taken in nonvault areas were below the limit. Ambient PCB levels are greatly increased during repair operations in all areas, 65 percent in excess of NIOSH exposure limits, ranging from 0.3 to 2.6 micrograms per cubic meter outside the vault, to 1.9 to 55.0 micrograms per cubic meter inside. Seventy nine percent of personal exposures exceeded the NIOSH criteria, ranging from 0.1 to 60.0 micrograms per cubic meter depending on the operation performed. At least 10 tetrachloro isomers were identified in the Askarel bulk samples. The authors conclude that workers involved in transformer maintenance and repairs are exposed to PCB concentrations many times the concentration recommended by NIOSH, and that repair operations do not seem to have a long term effect on indoor PCB concentrations.