The background and current status of the proficiency analytical testing (PAT) program were discussed. PAT is a collaborative effort of the American Industrial Hygiene Association and NIOSH designed to assist participating laboratories in improving their ability to analyze samples of occupational and environmental relevance. The program provides quality control reference samples to more than 1,300 participating laboratories in 17 countries. Each calendar quarter (round) reference samples consisting of mixtures of three of the metals cadmium (7440439), chromium (7440439), lead (7439921), or zinc (7440666), silica (14808607), either amosite (12172735), chrysotile (12001295), or manmade fibers, and one to three of the solvents benzene (71432), chloroform (67663), 1,2-dichloroethane (107062), methanol (67561), p-dioxane (123911), tetrachloroethylene (127184), toluene (108883), 1,1,1-trichloroethylene (71556), trichloroethylene (79016), o-xylene (95476), methyl-ethyl-ketone (78933), or methyl-isobutyl-ketone (108101) are mailed to the participating laboratories. The participants have been evaluated for each analysis by comparing their reported results against acceptable values established for each sample. PAT round 128 was sent out in January 1997 to 1,324 participating laboratories; 1,217 laboratories submitted results. In this round, 332 of 356 laboratories were rated proficient for the metals analysis, 77 of 78 were rated proficient for the silica analysis, 989 of 1,031 were rated proficient for the fibers analysis, and 288 of 320 were rated proficient for the solvents analysis. PAT round 129 was sent out in April 1997. The organic solvents in this round were 1,2- dichloroethane, tetrachloroethylene, and trichloroethylene, the metals were cadmium, lead, and zinc, the silica sample had a calcite matrix, and the fiber sample consisted of chrysotile with one manmade fiber.