Abstract The peripheral blood basophils of a patient with chronic myelogenous leukemia, basophilic phase (up to 85% of circulating leukocytes were mature basophils), and substantially elevated levels of urine histamine were studied in the electron microscope in an attempt to relate morphologic changes to possible mechanisms of mediator secretion. By use of criteria developed in earlier studies of allergic contact dermatitis, it was found that more than half of the mature circulating basophils were degranulated and less than 11% of them were fully granulated. Two patterns of basophil degranulation were recognized: (i) anaphylactic-type degranulation, involving fusion of adjacent granule membranes with each other and with the plasma membrane, permitting continuity between granules and the cell exterior, and associated with a concomitant loss of granule contents; (ii) a vesicular transport mechanism involving pinocytotic cytoplasmic vesicles. The anaphylactic mechanism probably accounted for the bulk of histamine release occurring in this patient, in contrast to volunteers with allergic contact dermatitis in whom the vesicular mechanism is more prominent.