We studied RAST and RAST inhibition tests to black GR, the most frequent sensitizer among several reactive dyes in our previous study, in order to evaluate the specificity of IgE antibodies to hapten or new antigenic determinants and the crossreactivity between two reactive dyes, black GR and orange 3R, in sera from four different sensitized workers. RAST inhibition studies with black GR-human serum albumin (HSA) conjugate discs demonstrated that black GR-HSA conjugates were the most effective inhibitors. Orange 3R-HSA conjugates, unconjugated forms of black GR and orange 3R were weak inhibitors in two patients and non-inhibitory in one patient, whereas they caused strong dose-dependent inhibitions in one patient. These results suggested that the IgE response to black GR-HSA conjugates might be heterogenous and the crossreactivity between two reactive dyes differed from one patient to another.