1. A gas-liquid chromatographic procedure for the determination of p-chlorophenoxyisobutyric acid (CPIB) has been elaborated and compared to the UV spectrophotometric procedure of Barrett and Thorp. 2. Both methods are specific when used to determine CPIB levels in normal sera from laboratory animals or humans treated with clofibrate (Atromid-S). Serum from patients treated with other drugs or abnormal sera from patients affected with a variety of diseases will often contain high and fluctuating levels of non-specific UV absorbing substances and this usually precludes the use of the UV procedure. 3. The GLC method is also applicable to the analysis of CPIB in urine samples.