[en] For 101 hospitalized patients where clinical symptoms of a glucosidine intoxication as a consequence of oral digoxin treatment were noted, serum digoxin levels were determined using a RIA kit of the Boehringer company (I-125 tracer, solid phase tube separation technique) on a fasted stomach prior to oral drug administration. An ECG was performed simultaneously and in addition kidney function parameters and electrolyte levels were determined. An anamnesis and clinical examination of the patients were also conducted. Interferences in the RIA method and gastro-intestinal illness resulting in changes in resorption behaviour were excluded. The group of patients showed collectively an average serum digoxin level of 2.9 +- 0.9 ng/ml with a range from 1.8 to 6.8 ng/ml, which was statistically significantly higher than the average value for 101 patients receiving long-term oral medication but without symptoms of glycosidine intoxication. According to the manufacture's specifications for the RIA kit, values > 2.0 ng/ml are considered as toxic, with values between 1.6 and 2.0 ng/mg falling into a ''grey zone''. A correlation was found between toxic serum values with changes in ECG pattern and subjective clinical symptoms of intoxication. A limitation of kidney function was responsible for the high serum glycoside levels in 62% of the patients. It was shown that the differing response of patients towards digitalis medication, above all in the case of long-term therapy, warrants an RIA determination of serum glycoside values in order to adjust the digoxin dose to individual requirements. (orig./MG)