We studied the latex agglutination inhibition method (the Biotrack system) for rapid measurement of plasma theophylline concentrations. Blood was collected from patients hospitalized for treatment of bronchial asthma and from healthy volunteers. The plasma theophylline concentrations of the samples were also measured by two other systems: fluorescence polarization immunoassay and high performance liquid chromatography (HPLC). When the results were compared, it was found that some combinations of Biotrack 516 units and cartridges afforded high plasma theophylline concentrations for some patients. Using HPLC, we found that the high values were not due to cross-reactivity with theophylline or caffeine metabolites. After careful investigation, the high values appeared to result from the characteristics of the individual Biotrack 516 units and cartridges used. Re-evaluation after improved materials were introduced into the system of Biotrack manufacturing yielded theophylline concentrations that correlated significantly with those obtained by HPLC.