1. Two main conjugates of CPIB (2-[chlorophenoxy]-2-methylpropionic acid) are present in the urine of subjects taking clofibrate. The metabolites can be separated by thin-layer chromatography (TLC). 2. Both conjugates are hydrolysed by dilute alkali, but only one is hydrolysed by the enzyme beta-glucuronidase. In eighty-five urine specimens this conjugate accounted for an average of 54.5% (range 25-70%) of the total CPIB, while 2.6-12.45% (mean 5.1%) was present as free CPIB.