Antibody quantification by EIA is possible without a standard curve. Following the so-called alpha method only one test dilution is used, the resulting absorbance is corrected and the IU/ml will be calculated by means of a mathematical formula. This new kind of a single point measurement was evaluated in seven independent laboratories by comparison with commercial EIAs using a standard curve or a titer calibration line. For the quantification of IgG against rubella virus this study comprised 1,480 individual samples and three comparison EIAs. For IgG against Toxoplasma gondii a total of 743 samples was evaluated in two comparison tests. The results obtained by the alpha method show a precision and accuracy more than sufficient for routine testings. Also the technical expenses and reagent costs were reduced. Prerequisites and limitations are discussed against the background of the problem of immune status definition.