The Proficiency Analytical Testing (PAT) program was discussed. PAT is a cooperative program of the American Industrial Hygiene Association and NIOSH that provides quality control reference samples of metals, silica (14808607), asbestos (1332214), and organic solvents to more than 1400 occupational health and environmental laboratories in the United States and foreign countries. The goal of the program is to assist the participating laboratories in improving their analytical skills. During each calendar quarter (round), samples are mailed to each laboratory. Returned results are analyzed and feedback is provided so that the laboratories can correct any analytical problems before the next round starts. The PAT program currently includes cadmium (7440439), chromium (7440473), lead (7439921), zinc (7440666), quartz (14808607), amosite (12172735), chrysotile (12001295), benzene (71432), carbon-tetrachloride (56235), chloroform (67663), 1,2- dichloroethane (107062), p-dioxane (123911), hexane (110543), methylene-chloride (75092), methylchloroform (71556), methylethyl- ketone (78933), methylisobutyl-ketone (108101), acetone (111659), tetrachloroethylene (127184), toluene (108883), trichloroethylene (79016), and o-xylene (95476). The performance of the participants is evaluated by comparing the data submitted for each sample against an acceptable performance limit determined by preselected reference laboratories. The results of PAT rounds 109 and 110 conducted in April 1992 and July 1992, respectively, were discussed.