The Proficiency Analytical Testing Program (PAT), established through the American Industrial Hygiene Association and NIOSH was described, and recent evaluations examined. PAT is designed as a quality control program for occupational health and environmental laboratories in more than 15 countries. Reference samples containing metals, silica, asbestos, and organic solvents are sent to participating laboratories each calendar quarter, who analyze the samples and the results of these analyses are compared to results from PAT reference laboratories. Participating laboratories are rated on their performance in the last year, on individual contaminants and overall performance. A table detailing the results of 1,448 laboratories who submitted results in PAT round 105, in April 1991, was presented. Performance ratings for July 1990 through June 1991 demonstrated 15.4% of laboratories were rated nonproficient for metal contaminants, 2.7% for silica, 6.4% for asbestos, 14.8% for organic solvents, and 10.2% were rated nonproficient overall. The most recent round, number 106, was sent out on July 1, 1991, and included benzene, o-xylene, toluene, cadmium, chromium, lead, silica with a talc background and amosite asbestos.