Three workers with TDI hypersensitivity were evaluated for IgE antibodies to TDI over a period of 13 months. A radioallergosorbent test (RAST) system was employed using p-tolyl(mono)isocyanate-human serum albumin antigen covalently bound to cyanogen bromide-activated paper discs. IgE antibody titers were consistently elevated in two individuals who experienced several bronchial hypersensitivity responses to TDI during the study period. The responses were either solely asthmatic or asthmatic accompanied by cutaneous hypersensitivity reactions. By contrast, antibody titers in a third subject who had not experienced any hypersensitivity reactions during the study period continually decreased, falling to insignificant levels after 12 months. In the absence of renewed TDI exposure, sensitive workers may have titers indistinguishable from those of workers exposed to TDI but without sensitivity to the chemical.