A retrospective study was made of workers in a toluene disocyanate plant using an immunologic test procedure developed at the NIOSH laboratories. Case histories of 32 workers in seven response groupings are presented. Sensitization was found to correlate with the frequency and severity of significant exposures to TDI. Immunologic changes producing tolerance to TDI exposure also resulted in negative response to the antigen challenge tests. Some individuals exposed to TDI may have protective antibodies but still experience bronchoconstriction.