The analysis of theophylline and other xanthine derivatives in biological fluids was examined by high-pressure liquid chromatography (HPLC) and spectrophotometric (Schack and Waxler) methods. An HPLC analysis technique involving precipitation of serum proteins using trichloroacetic acid, injection of the supernatant onto a strong cation exchange column and detection using ultraviolet absorbance provides a nonpolluting, micronized, rapid and reasonably sensitive and specific method for measuring serum theophylline concentrations during clinical pharmacokinetic monitoring. Pre-extration of xanthines with chloroform increases the sensitivity and specificity of the assay. Good agreement was found in the assay results between the spectrophotometric and HPLC assays in 23 of 25 patient specimens, but occasional severe interferences were found with the former technique.