The occurrence of isohemolysis without iso-agglutination has not been observed in man. Experimentally, however, Ottenberg and Thalheimer 1 have produced isohemolysis in cats whose serum did not agglutinate. This was done by transfusing blood from one cat to another, whose blood before the transfusion was entirely compatible with that of the first. This has not yet been shown to be true for man. The production of isohemolysins and the iso-agglutinins in human blood by means of transfusion is demonstrated by the following case reports: Case 1. —J. H., a girl, aged 18 months, a first child, born of healthy parents, was admitted to the hospital with signs and symptoms indicating a nasopharyngitis and bilateral acute otitis media. Both ear drums were incised, and much pus was evacuated. One week after admission there were signs of involvement of the mastoid process, and it was decided to perform a blood transfusion because