A low-molecular-size antigen, LM2, which possesses a significant activity to elicit histamine release, was isolated from an ultrafilterable (Mr-cutoff < 10k) fraction of a mite extract containing feces by consecutive chromatography on Ultrogel AcA 54 and Sephadex G-25. The isolated antigen was still heterogeneous glycoproteins, giving a smeary band around pI 3-5 on an Ampholine PAG plate, with molecular weights of 6-8k and 4k on SDS-PAGE and Sephadex G-50 gel filtration, respectively. Chemical treatments of the antigen suggested that the epitope responsible for the elicitation of histamine release resided in the protein moiety. The antigen had activities to provoke conjunctival congestion and histamine release in mite-allergic patients, but not immunogenicity by itself. The antigen competitively inhibited the reactions of HM1, HM2, and HM3 fractions to corresponding antisera, but did not cross-react with anti-Der f I or anti-Der f II sera.