A basic and clinical study was carried out to evaluate the usefulness of the Shionoria Specific IgE (SIST) kit for false cypress allergen. The test materials were sera from 152 patients who visited our clinic with symptoms of nasal allergy during the period when Japanese cedar and false cypress pollens are prevalent in the air. The standard curve drawn on the standard serum, which was part of the test kit, was linear. Dilution, inhibition and absorption tests were all satisfactory. Reproducibility studies, different-day, and inter-performers, were all excellent. Nonallergic sera and umbilical cord sera showed less reactivity than the reference serum which is classified as class 0. The sensitivity of the SIST kit for False cypress was 89.9%; specificity 86.3%, and concordance rate 88.2%. The SIST test results for the sera well correlated with the results given by skin tests. These results are good evidence of the excellence of this kit. It was of interest that in the inhibition test, some false cypress positive sera were blocked with Japanese cedar extract as well as with false cypress extract, while other false cypress positive sera were blocked only with false cypress extract. Similar results were obtained in the absorption test. This suggests that the antibodies of some patients recognize a portion of the false cypress allergen which is common to a particular portion of the Japanese cedar allergen.