Data are presented correlating 22 determinations of the histamine content of blood in patients with chronic granulocytic leukemia with (1) the basophils (2) the other myeloid granulocytes. Expressing the histamine in each case on the basis of the amount in 108 granulocytes, there is a readily evident positive correlation with the basophil percentage and a negative correlation with the percentage of other myeloid elements. This is strikingly apparent in instances where blood with a very high percentage of basophils was analyzed. The data indicate that in chronic granulocytic leukemia, the basophil is predominantly and, perhaps exclusively, responsible for the marked elevations in blood histamine. Inferentially, it appears probable that this cell type, though small in numbers, may be the principal carrier of histamine in non-leukemic blood.