Four patients with asthma due to tolylene di-isocyanate (TDI) were tested by occupational type exposure to a polyurethane varnish and its TDI activator'. Late asthmatic reactions were given in two patients, the third gave an immediate reaction and the fourth gave a dual, immediate followed by late asthmatic, reaction. The reactions in the last two patients were inhibited by disodium cromoglycate. Two patients encountered the TDI in fumes from soldering polyurethane coated wire. The possibility of sensitization in conditions of limited exposure requires more frequent consideration as none of the four subjects tested gave histories of heavy exposure.