Leukocytes were labelled by intravenous injection of tritiated thymidine (3H-thymidine) in dogs to discover the source of the increased number of neutrophils in the circulating blood after injection of histamine in beeswax. Dogs with normal hemograms were given 1.0 mCi/kg of 3H-thymidine alone, and in different sequences, with histamine in beeswax. When 3H-thymidine was given during maintained histamine leukocytosis, labelled granulocytes appeared in and disappeared from the blood earlier than in control tests and the number of labelled cells was greater in the histamine-treated animals. Administration of histamine in beeswax 3 days after injection of 3H-thymidine also induced the premature appearance and disappearance of labelled neutrophils in the circulating blood. It was concluded that leukocytosis induced by the chronic action of histamine is due to 1) stimulated proliferation and differentiation of neutrophil precursor cells in the bone marrow and 2) the release of mature leukocytes from the bone marrow.