1. (14)C-L-histidine was given i.v. to one normal subject and two patients with chronic myelocytic leukaemia. Urinary excretion of histamine and two of its metabolites methylhistamine and methylimidazoleacetic acid, total as well as (14)C-labelled, was measured, as well as blood (14)C-histamine. In addition the total urinary and pulmonary elimination of (14)C was followed.2. Total (14)C elimination was high during the first days, then declined slowly except for a plateau at the 10th-14th day in the two patients. There was a measurable elimination even after some months.3. (14)C-histamine appeared in the blood of the leukaemic patient, whereas in the normal subjects the values were hardly measurable.4. The leukaemic patients excreted much more of the two (14)C-labelled histamine metabolites than the normal subject. The difference in excretion of the (14)C-labelled metabolites was largest around the 12th day after the infusion of (14)C-L-histidine.5. The results indicate that the leukaemic patients formed at least 20 times more histamine daily than the normal subject.