A radioimmunoassay for histamine was used to measure histamine and 1-methylhistamine (MH) in human urine samples. The detection limit of this assay was 2 ng/ml for histamine and 0.5 ng/ml for MH. Cation exchange high performance liquid chromatography (HPLC) on a Bio-Gel TSK SP-5 PW column with gradient elution was capable of separating histamine from its precursor L-histidine and its metabolites MH and 1-methylimidazole acetic acid (MIAA). The concentration of MH-immunoreactive material in healthy volunteers with no dietary restrictions was 202 ± 92 μg/24 h (mean ± SD; n = 14). The excretion of MH-like material, expressed as micrograms of MH per 24 h, was not significantly different before or after the intake of histamine-rich food: 217 ± 88 vs. 276 ± 135 μg/24 h (n = 10). However, in urine samples collected in individual fractions, the levels of MH immunoreactivity were significantly increased after a histamine-rich meal in comparison to the corresponding fractions which were taken a day earlier at the same time intervals after a histamine-low diet (p < 0.03). HPLC characterization of MH immunoreactivity revealed the presence of histamine, MH and a compound with the same retention time as MIAA. The ratio of histamine, MH and MIAA in controls without dietary restrictions, as determined by HPLC analysis, was 50 ± 6, 47 ± 5 and 3 ± 1, respectively. After a histamine-rich meal the ratio was 97 ± 2 for histamine, 1 for MH and 2 ± 1
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