The background and current status of the Proficiency Analytical Testing (PAT) Program were reviewed. The PAT Program, administered by the American Industrial Hygiene Association and NIOSH, provided quality control reference samples to numerous occupational health and environmental laboratories. Metal, silica, asbestos (1332214) fibers, and organic solvent samples were sent to participating laboratories on a quarterly basis. The analytical results of the participating laboratories were compared to those of the selected reference laboratories. The acceptable performance limits were +/-3 standard deviations of the statistically treated reference laboratory data. Laboratories were rated based on performance in four rounds of the PAT program. PAT Round 127 was sent to 1,331 participating laboratories in October 1996. A total of 1,212 laboratories submitted results. The relative standard deviations of the reference laboratories ranged from 3.2 to 5.7% for metals, 20.0 to 27.6% for silica, 27.7 to 34.2% for asbestos/fibers, and 3.8 to 8.4% for organic solvents. The number of outliers varied from 27 to 42 laboratories for metals, 1 to 5 laboratories for silica, 31 to 91 laboratories for asbestos/fibers, and 15 to 37 laboratories for organic solvents. Based on PAT Rounds 124 through 127, 91.8% of 356 laboratories were rated proficient for metals, 98.7% of 79 laboratories were rated proficient for silica, 95% of 1,027 laboratories were rated proficient for asbestos/fibers, and 92.6% of 311 laboratories were rated proficient for organic solvents. Results of PAT Round 128, in which methyl-ethyl-ketone (78933) and methyl-isobutyl-ketone (108101) have been included for the first time, are forthcoming.