<i>Background:</i> Measurement of the soybean aeroallergen in Barcelona and other cities where soybean is unloaded is of increasing importance in controlling population exposure and evaluating the influence of such exposure on the persistence of asthma symtpoms. <i>Objective:</i> The aims of the study were: (1) to standardize an amplified ELISA inhibition method for the quantification of soybean aeroallergen and (2) to compare this method to a previously described RAST inhibition method. <i>Methods and Results:</i> An amplified competitive ELISA inhibition method with a biotin-streptavidin system was carried out using a pool of sera from soybean-sensitized patients. The results were expressed as U/ml using a low-molecular-mass soybean allergen as reference standard. Reproducibility was calculated by statistically comparing the slope of the regression lines of the standard curve of 4 consecutive assays and by determining the coefficient of variation (CV) of the percent inhibition data for each point of several independent standard curves, each from the same assay (intra-assay) and also from a separate assay (inter-assay). No significant differences in the slopes were obtained by analysis of covariance (ANCOVA) F = 1.04. The CV between assays varied between 4 and 22% (for the assay range used in the reference standard) and was greater than the CV within assays (5–10%). Only values with a CV(%) smaller than 20% were considered acceptable. 78.5% of the samples satisfied this criterion. The RAST inhibition and ELISA inhibition methods were compared by difference plots from the values of 338 air filter eluates. The intraclass correlation coefficient was 0.456 (p < 0.001). After the results of both methods were classified as lower and higher than 165 U/m³, the kappa index was 0.46 (p < 0.001). <i>Conclusions:</i> The data obtained in the present study are comparable to those reported from other similar immunoassays. Moreover, despite the difficulty in comparing air-sampling values from different laboratories, the kappa index may be taken to represent fairly good agreement beyond chance between both methods. All these data demonstrate that the present immunoassay is useful for measuring airborne soybean aeroallergens and can also be applied to evaluate the relationship between exposure and the development of asthma symptoms.