Sputum samples from patients with chronic bronchitis were assayed for histamine content using a bioassay, and/or fluorometric assay and single or double radioenzymatic techniques. Comparison showed that all the assays produced significant differences in the measurement of sputum histamine. Similarly, it was observed that the assays all showed significant differences in the measurement of histamine released from human basophils. In general the fluorometric assay give the highest values for histamine in sputum and the double-isotope radioenzymatic assay the lowest. In contrast, no differences were found between the bioassay, fluorometric assay and the single-isotope radioenzymatic assay for the measurement of histamine added to Tyrode's solution. These results suggest that in biological samples some of the assays may additionally be measuring metabolites of histamine or other components present in the samples and underline the need to measure the levels of histamine and its metabolites separately in sputum.