As molecular size of the most major allergens of house dust mite(HDM) are known to be less than 30 kD, allergic immune reactions may be different according to the molecular fractions of HDM allergens. In this study, we fractionated D. pteronyssinus(D. pt) antigen according to its molecular weights,' fraction I(FI), II(FII, more than 30 kD), III(FIII, less than 30 kD), and evaluated the differences of allergic immune responses in 40 atopic patients. FIII(A/H ratio 2.44+-0.61) elicited stronger response on intradermal skin test than FI (0.76+0.10, p<0.001) and FII(0.88k0.12, p <0.001). The immunoblot analysis revealed that the nine out of 10 atopics had IgE binding bands with 15 kD protein of FIII which were not found in FI and FII. The geometric mean of absorben value of specific IgE to FIII was 0.067, which was higher than that of FII (0.013, p<0.001), with no difference from that of FI(0.042). The skin reactivity to whole D. pt antigen correlated with FIII specific IgE(r=0.3862, p<0.01), not with FI (r=0. 2605, p=0.07) and FII specific IgE(r=0. 1103, p=0.45). FI specific IgG levels in atopic sera(n=40) were significantly higher than that of FII and FIII in all IgG subclasses (p<0.001). Both atopic(n=4) and nonatopic (n=4) groups showed multiple IgG, IgG1, and IgG2 binding ba,nds at high molecular proteins in immunoblot analysis. The prevalence and density of the IgG and IgG1 binding bands to 15 kD were higher and more intense in atopics than those of nonatopics. In conclusion, we suggest that fractionated antigens of D. pteronyssinus had different antigenicities and that the allergic irnmune responses to HDMs in atopics and nonatopic are different according to molecular fractions of HDM.