To study the preventive effect of supplementary iodine on interdigital phlegmon, 50 Swedish red and white calves weighing approximately 100 kg each were given 1 mg ethylenediamine dihydroiodide (EDDI) per kg bodyweight daily. Fifty calves in the same herd were untreated controls. As a result of severe side effects the experiment was interrupted after five weeks. No case of interdigital phlegmon was observed, but 31 calves in the EDDI group and one in the control group were treated for respiratory disease. Serum protein bound iodine increased on average from 68.0 to 213.0 micrograms/litre in the EDDI group compared to an increase from 59.9 to 70.7 micrograms/litre in the control group.