Clinical pathologists occasionally experience difficulty in counting erythrocytes because of unusually rapid rouleau formation. In extreme cases it may be impossible to count red cells because of immediate clumping before the blood is mixed with diluting fluid. This condition often presents difficulties when the occasion arises for a blood transfusion. Prompt rouleau formation of all foreign erythrocytes suspended in the patient's serum causes confusion and inability to select donors until the nature of the underlying mechanism is understood. A case in which both of these difficulties were encountered directed us to inquire into the underlying causes and led to the following observations on a highly pathologic plasma. Abnormally rapid rouleau formation has been observed for many years and is known by various names —autohemagglutination, panagglutination, pathologic panagglutination and pseudo-agglutination. It is sharply distinguished from true iso-agglutination and from auto-agglutination, which is affected by changes in temperature. The earliest comments on