Assay of lectins in mistletoe preparations was based on an improved and validated version of ELLA (enzyme-linked lectin assay) to meet the requirements given in the guidelines for drug tests. The monoclonal antibody used has more than 90% cross reactivity with the three known mistletoe lectins, so that total lectin content is determined with much greater accuracy. With the detection and quantitative analysis limit below 5 ng/ml and a linear measuring range of 5-50 ng/ml, dosages in therapeutic range can be assayed. Tests to establish the accuracy of the analytical method showed that up to 26% of lectin activity is suppressed by other constituents of the extract, so that the recovery must be taken into account. The recovery increases following ultrafiltration to remove low-molecular constituents. Analysis for precision gave a variation coefficient of < or = 7.7% and a confidence interval < or = 5.7% (p = 0.05) for total lectin concentrations of approx. 250 ng/ml. This level of precision, which is good for an immunologic assay, makes it possible to standardize mistletoe preparations.