A simple method for obtaining living leukocytes from human peripheral blood is presented. It consists of the addition of fibrinogen to heparinized blood. Essentially all the leukocytes remain in plasma suspension and essentially all the erythrocytes are sedimented out at the end of one hour.

ACKNOWLEDGMENT The authors wish to express their thanks to Miss Alma Manieri, who did all the blood counts for this study, and to Mr. Antol Herskovitz, who prepared the photomicrographs.