A purified preparation of a major allergen of Japanese cedar pollen, sugi basic protein (SBP, Cry j I), was separated into 5 subfractions of 50-45 kDa. All of the SBP subfractions were confirmed to be reactive to IgE antibodies from patients with Japanese cedar pollinosis, and also to mouse anti-SBP monoclonal antibodies. The sequences of 20 N-terminal amino acids of these 5 subfractions were found to be identical. Peptide mapping analyses of the SBP subfractions showed similar patterns, with some differences which might in part be due to the existence of an N-linked carbohydrate chain. The N-terminal amino acid sequence of SBP was identical to the reported sequence of an allergen of mountain cedar which vegetated in North America.