The prevalence of work-related wheeze and breathlessness was measured in factory employees manufacturing flux-cored solder. The flux contained colophony which was heated in the production process, exposing the workers to colophony fumes. Measurement of colophony in the breathing zone defined three grades of exposure with median levels of 1.92 mg/m3 (six subjects), 0.02 mg/m3 (14 subjects), and less than 0.01 mg/m3 (68 subjects). All but two workers in the first two groups, and 90% of a random sample of the last group, were studied. Occupational asthma was present in 21% of the higher two exposure groups and 4% of the lowest exposure group. Mean values of FEV1 and FVC fell with increasing exposure. The prevalence of upper and lower respiratory symptoms was only one-third to a half that found in a previous study of shop floor electronics workers, whose work raised the flux to a higher temperature and produced higher concentrations of colophony fume. Total IgM levels were higher in the solder manufacturers than in unexposed controls, and were higher still in the electronics workers. The solder manufacturers were exposed to colophony fumes at 140 degrees C, below the temperature at which the resin acids decompose, supporting the hypothesis that it is the whole resin acids rather than decomposition products which cause occupational asthma. The threshold limit value should be based on the resin acid content of the fume, and not the aldehyde content as at present. The survey suggests that sensitisation will not be prevented unless exposure is kept well below the present threshold limit value.