The background and present status of the Proficiency Analytical Testing (PAT) Program of the American Industrial Hygiene Association (AIHA) and NIOSH were discussed. The program provided quality control standards to occupational health and environmental laboratories, evaluated analytical performance and assisted in performance improvement. Program rounds consisted of calendar quarters. Samples were sent to participating laboratories, and analytical results were evaluated and repeat samples sent, if necessary, for correction of problems. Samples included metal, silica (7631869), asbestos (1332214) and organic solvent standards. Sample rotations included three of four metal samples, three of 15 organic solvents and amosite (12172735) or chrysotile (12001295) asbestos. Metal, silica and asbestos samples were provided on filters and organic solvents on charcoal tubes. Four different concentrations and a blank were provided for each. Reference laboratories were preselected based on proficiency rating in the last PAT evaluation of all contaminants and accreditation by the AIHA (United States laboratories only). Performance evaluation was based on reference laboratory values. Data were acceptable if values were within three standard deviations of the mean and were outliers otherwise. Performance for individual contaminants was proficient if all results were reported and were acceptable for the last two consecutive rounds or if at least three quarters of results reported in the last four consecutive rounds were acceptable. Overall performance was proficient if at least two thirds of individual contaminants were analyzed proficiently and was nonproficient if any contaminants were rated nonproficient for more than 1 year. In rounds 101 to 104, performed from April 1990 through March 1991, 1409 laboratories were rated. Overall nonproficiency rating was 6.7%. Nonproficiency ratings were obtained by 11.7% of laboratories for metal samples, 4.4% for silica, 3.4% for asbestos and 14.3% for organic solvents.