Summary: Immune histamine release from rabbit platelets is characterized by a lack of antibody fixation, be it obtained in the “early” or “late” stages of immunization. The immune histamine release phenomenon does not interfere with the K+ pumping site on the cell membrane. The evidence indicates that the immune platelet response results in irreversible damage with protein leakage into the extracellular medium. This mechanism of release differs from platelet aggregation due to adenosine diphosphate. The mechanism of histamine liberation from rabbit platelets differs from other types of allergic histamine release. It may be characterized as a serum-dependent reaction mediated by homologous antibody and resulting in cytotoxicity without prior fixation of the antibody onto the target cells.