Several species of storage mite have been shown to cause IgE-mediated sensitization. Monoclonal antibodies (mabs) have been raised against the storage mite Lepidoglyphus destructor, and one mab (42B6) immunoprecipitated a band of 39 kDa of L. destructor. To search for the frequency of IgE antibodies in patient sera against this allergen component, we modified a radioimmunoassay (RIA), 42B6 being used as capturing antibody. Twenty of 43 sera from farmers who were RAST positive to L. destructor (46.5%) were scored as positive in this assay. There was a moderate degree of correlation between positive RAST results to L. destructor and the 39-kDa allergen. Among 14 sera from Stockholm residents positive in RAST to both Dermatophagoides pteronyssinus and L. destructor, six detected the 39-kDa allergen component. However, three sera from urban subjects lacking IgE antibodies against both mite species also had IgE antibodies against the 39-kDa allergen. The present study demonstrates that the 39-kDa allergen of L. destructor is an important allergen, and that rural as well as urban subjects can become sensitized.