The EMIT technique for theophylline measurement as adapted to the Abbott Bichromatic Analyzer (ABA-100), was compared to two other established procedures. EMIT results correlated well with those obtained with high pressure liquid chromatographic (HPLC) (r = 0.995, N = 54) and spectrophotometric (UV) (r = 0.95, N = 37) methods when the patient samples used were free of apparent interferences in the comparison methods. On samples exhibiting interferences with HPLC and/or UV methods, the EMIT assay gave the same results as a gas-liquid chromatographic procedure which showed no interferences. In addition, theophylline metabolites, xanthine analogs and many commonly administered drugs did not interfere with the EMIT theophylline assay. An interassay coefficient of variation of 3.5-6.5% was demonstrated. Our data indicate that the EMIT technique exhibits adequate precision and specificity for routine theophylline determinations. Unlike the HPLC or UV procedures, we have not encountered any interferences with the EMIT technique.