1. The clinical histories of 11 patients with a severe and hitherto undescribed type of reaction to in vitro compatible whole blood are presented. These patients all experienced beneficial responses to therapy with washed red blood cell transfusions. The technic of washing the blood is outlined.
2. In 5 of the patients, sensitivity to a heat-labile constituent of fresh normal plasma was demonstrated. In the remaining 6 patients, although actual sensitivity to plasma was not proved, transfusions of washed red blood cells were well tolerated and were therapeutically effective.
3. These uncommon reactions to the plasma in whole blood are differentiated from the usual pyrogenic, allergic or hemolytic plasma reactions by their ease of repetition in the absence of pyrogens, by lack of the usual allergic manifestations, and by the failure to demonstrate hemoglobinemia or hemoglobinuria.
4. A simple diagnostic test ("plasma provocative test") is described which appears to be of specific value in the differentiation of the reaction.
5. It is suggested that in any instance of severe transfusion reaction to apparently compatible whole blood this type of sensitivity to the donated plasma should be borne in mind as a possible cause. The plasma provocative test should then be tried. If positive, the transfusion of well washed red blood cells will probably be effective and in some cases may be of life-saving importance.