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An occupational type exposure-test to toluene di-isocyanate (TDI) for the diagnosis of specific respiratory sensitization is proposed. The subjects were tested by exposure to a two-pot polyurethane varnish with and without TDI activator, using a paint-spraying system under standard and controlled conditions, so as not to exceed the threshold limit value of TDI. Forced expiratory volume per second and specific airway resistance (plethysmographic method) were measured before each exposure and during the following 24 h. 47 subjects with clinical symptoms of asthma were examined during their hospital care. For the purpose of this study they were divided into two groups: 42 workers exposed to TDI and 5 not exposed. The preliminary control test, using polyurethane varnish without TDI activator, was always negative and no significant changes in ventilatory parameters were recorded. The spray-painting test, with the same varnish and TDI activator, gave positive responses in 35 out of the 42 exposed workers and negative findings in the group not exposed. This method permits to formulate an accurate diagnosis of TDI respiratory sensitization in the majority of cases (83%). Immediate asthmatic reactions were frequently observed in the exposed subjects with high acetylcholine reactivity. Ten subjects showed a late bronchial reaction (3 h or more after TDI exposure).