Two antigens, both containing tolyl groups, were compared for ability to detect IgE antibodies in workers hypersensitive to toluene diisocyanate. One antigen, formed by reaction of p-tolyl isocyanate with human serum albumin, detected antibodies in each of ten hypersensitive workers. A second tolyl antigen, formed by reaction of p-toluoyl chloride and human serum albumin, and therefore lacking isocyanate linkages, detected antibodies in seven of the ten workers. In RAST assays, sera from toluene diisocyanate-sensitive workers demonstrated higher binding to human serum albumin-coated control discs than did sera from non-sensitive workers. The significance of this binding in calculating tolyl-reactive antibody titres is discussed.