One hundred and thirty-six subjects with asthma and/or hay fever were surveyed for possible sensitization to the Bacillus licheniformis protease present in the commercial antigenically identical enzymes (Alcalase® and Maxatase®) used in household detergents. Prick-test reactions reported as minimally positive were given by one subject to both commercial enzyme preparations at the same standard non-irritating concentrations and by four others to one but not the other antigenically identical enzyme. Two of the five, and an additional seven, gave weak prick tests to the purified enzyme (Koch-Light Protease) at test protein concentrations 100 times stronger and to which non-specific irritation occurs. The findings are strongly against interpretation of these reactions as positive and were on no occasion comparable with those in sensitive workers. The skin-test reactions are attributable to non-specific effects. Neither enzyme-specific RAST IgH tests nor clinical histories suggested clinical sensitivity. Enzyme-specific RAST IgE for eighty-eight subjects over a period of 5 years showed no correlation with exposure. None had developed specific IgH levels like those of sensitized workers, and their RAST levels did not differ significantly from cord blood, unexposed subjects and prick-test-negative workers. The eight with the largest increase in specific IgE were prick-test-negative and their clinical reactions to enzyme detergents were comparable to those of non-enzyme detergents. There is no evidence from this study that exposure to modern, non-dusty household enzyme-containing detergents is likely to lead to clinical sensitization.