Urinary hexahydrophthalic acid (HHP acid) levels were determined in 20 workers occupationally exposed to hexahydrophthalic anhydride (HHPA) air levels of 11-220 micrograms/m3. The levels of HHP acid in urine increased rapidly during exposure and the decreases were also rapid after the end of exposure. The elimination half-time of HHP acid was 5 h, which was significantly longer than in experimentally exposed volunteers, possibly indicating distribution to more than one compartment. There was a close correlation between time-weighted average levels of HHPA in air and creatinine-adjusted levels of HHP acid in urine collected during the last 4 h of exposure (r = 0.90), indicating that determination of urinary HHP acid levels is suitable as a method for biological monitoring of HHPA exposure. An air level of 100 micrograms/m3 corresponded to a postshift urinary HHP acid level of ca. 900 nmol/mmol creatinine in subjects performing light work for 8 h. Percutaneous absorption of HHPA was studied by application of HHPA in petrolatum to the back skin of three volunteers. The excreted amounts of HHP acid in urine, as a fraction of the totally applied amount of HHPA, were within intervals of 1.4%-4.5%, 0.2%-1.3%, and 0%-0.4% respectively, indicating that the contribution from percutaneous absorption is of minor importance in a method for biological monitoring.