Recently large amounts of Wallemia sebi, a species of osmophilic fungi, have been detected in house dust by low water activity media. The allergenic activity of W. sebi was examined by skin prick tests and radioallergosorbent tests (RAST) in 74 asthmatic patients (mean age 11.7, range 6-32). Aspergillus fumigatus and house dust were used for comparison. In the skin prick tests, W. sebi extract, A. fumigatus extract and house dust extract elicited positive reactions in 4 (5.4%), 4 (5.4%) and 51 (68.9%) patients, respectively. RAST showed positive results in 14 subjects (18.9%) for W. sebi extract, in 8 (10.8%) for A. fumigatus extract and in 59 (79.7%) for house dust extract. These results indicated that some asthmatic individuals showed immediate type hypersensitivity to W. sebi, which means this fungal species may be important as a causative agent in atopic diseases. Additionally, the authors measured W. sebi-specific IgG by enzyme-linked immunosorbent assay in asthmatic (n = 28) and non-atopic patients (n = 28). W. sebi-specific IgG was found in sera from all subjects in each group. W. sebi-specific IgG in asthmatics (mean +/- SD = 0.686 +/- 0.160) was significantly higher than that in non-atopics (mean +/- SD = 0.572 +/- 0.188) (p less than 0.01).