This report deals with a rare property in the blood of a patient whose serum showed an iso-agglutinin of moderate activity, which agglutinated about 80 per cent of the bloods of her own group. In view of the fact that this agglutinin tended to disappear after an interval of several months and the fact that this agglutinin gave an equally strong reaction at 37 and 20 C., it would seem to resemble agglutinins resulting from iso-immunization following repeated transfusions. This phenomenon is readily reproduced in some species (cattle, chickens, rabbits), by several repeated transfusions, but in the case of man only two clearcut instances of such iso-immunization to cellular elements are described in the literature.1 The case to be described differs from these in that the immune iso-agglutinin must have been stimulated by a factor other than repeated transfusion. The nature of this factor becomes evident from a summary