Virological and serological methods were used in examination of 28 patients 
suffering from subacute thyroiditis de Quervain. Attempts to isolate a presumed 
viral agent from 8 patients were performed by inoculation of serum, urine, and 
aspiration biopsies of thyroid glands taken at different stages of the illness, 
into tissue cultures of different types of human and animal cells. Recovery of a 
cytopathic viral agent on cells of a rabbit lung continuous line was successful 
in 5 cases. Serological cross reactions exist between the isolated viruses and 
patient serum but not with serum of healthy people. Cases with the acquired 
illness and positive antibodies against the isolated viruses who had been in 
close and prolonged contact with patients suffering from subacute thyroiditis de 
Quervain were also investigated.