The results of round 113 of the Proficiency Analytical Testing Program (PAT) administered by the American Industrial Hygiene Association, the NIOSH, and the Centers for Disease Control and Prevention conducted in April, 1993 were reported. In addition to the standard proficiency testing performed each calendar quarter, the solubility of polyvinyl-chloride filters using tetrahydrofuran for silica (14808607) analysis was studied in this PAT round. All four types of filters tested were found to have acceptable levels of solubility. Of the 1,324 laboratories rated in this PAT round over 90% were rated overall proficient. An examination of the ratings of 1,374 laboratories based on the results of the last four PAT rounds demonstrated that the largest percentage of nonproficient ratings involved the analyses of organic solvents, followed by metals, and asbestos samples.