High performance liquid chromatographic (HPLC) and enzyme immunoassay (EMIT), methods for quantitating the bronchodilator theophylline concentrations in human serum were evaluated. For both methods, standard curves were linear over the therapeutic range of serum concentrations. Precision was acceptable, with coefficients of variation < 9%. The HPLC assay was slightly, but not significantly, more precise. No interference was noted in either method by caffeine, theobromine, diphylline, 8-chlorotheophylline, or by metabolites such as 1-methylxanthine and 3-methylxanthine, or by potentially interfering compounds such as urea and uric acid. The 2 methods correlated well, with a correlation coefficient of 0.98%. The enzyme immunoassay was superior in terms of lower costs, greater ease of performance and potential for automation.