Naturally occurring low molecular weight biogenic amines and amino acids were analyzed for interference in the fluorometric histamine assay. Three mechanisms of interference were detected, mimicking of histamine, suppression of histamine fluorescence and generation of increasing histamine-like fluorescence during excitation or preirradiation of the o-phthaldialdehyde condensation product with daylight or UVA light. The latter effect was seen only with amino acids. Rat peritoneal lavage fluid contained no fluorescence-suppressing substances, but there were considerable amounts of histamine-mimicking compounds which could not be digested with a diamine oxidase, and of substances generating increasing histamine-like fluorescence when exposed to UV light after condensation with phthaldialdehyde. Although butanol extraction was highly effective in selecting histamine over interfering compounds, it was not sufficient to avoid interference. In contrast, Dowex 50 W-X8 ion exchange chromatography added little to separating histamine and interfering substances. Precautions are recommended to avoid the different types of interference when biological samples are analyzed for histamine content with the fluorometric assay.