“…Evidence has been presented for a leukopoietic or leu kocytosis-inducing factor which has been obtained from the plasma of ani mals undergoing repeated leukapheresis or receiving bacterial endotoxin (71), by the study of isolated rat femora and spleens (72,73). There are problems in the interpretation of this data since it is difficult to separate the observed effects due to the postulated factor from effects which might be due to a granulocyte-mobilizing agent, such as endotoxin or etiocholanolone.…”