The affinities of some polar and non-polar digoxin and digitoxin metabolites to the related antibodies are largely dependent on the structure of their genin parts. Metabolites with unchanged genins show high cross reactivities towards their related antibodies when compared to the original immunogenic cardenolides. Towards the digoxin antibody the following cross reactivities were found: digoxin-16'-glucuronide 50%, digoxigenin 89.5%, and digoxigenin-3-glucuronide 85%. Values of the same order of magnitude were found with the corresponding digitoxin metabolites and the digitoxin antibody. In contrast, there is a marked decrease in cross reactivity caused by only minor changes in the genin part. With the digitoxin antibody the following cross reactivities are found: digoxin (i.e. 12-OH-digitoxin) 9.1%, 3-epi-digitoxigenin 5.6% and 3-epi-digitoxigenin-3-sulphate 1%. Bcause 12-hydroxylation has been reported to be one of several possible ways of digitoxin metabolisation in man, from a theoretical point of view erroneous results in the clinical digitoxin estimation by radioimmuno-assay are possible. In the case of all of the other metabolites the alterations in positive inotropic activity and cross reactivity run largely parallel.