House dust mites are the most common cause of allergic sensitization in respiratory allergic patients in the world. Tyrophagus putrescentiae (TP), which was followed by Dermatophagoides farinae (DF) and Dermatophagoides pteronyssinus (DP), has been reported as the third most common house mite in Korea. We previously reported that many respiratory allergic patients had become concomitantly sensitized to DP, DF and TP. The aims of this study were to identify the allergic components of TP and to evaluate the cross-reactivity between TP and DP. The allergenic components of TP and DP extracts were determined with SDS-PAGE and IgE immunoblotting analysis. The cross-reactivity as evaluated by ELISA inhibition and inhibitory immunoblotting experiments. According the SDS-PAGE, the protein components of the two extracts were somewhat different, although a few components displayed identical molecular weights. The 18 kD protein of TP was the most prevalent allergen in the sera of patients sensitized to TP and DP. Both of the maximum inhibition percentages of optic densities of TP-specific IgE in ELISA with TP and DP extract were 100%, respectively and the 50% inhibitory dose (ID50) of TP extract and DP extract were 0.01 micrograms/ml and 0.02 micrograms/ml, respectively. Maximum inhibition of optic densities of DP-specific IgE in ELISA with TP and DP extracts were 29% and 100%, respectively and the ID50 of DP extract was 0.007 micrograms/ml. On inhibitory immunoblotting of DP specific IgE, 5 micrograms/ml of TP extract completely inhibited 16 kD without inhibiting the other allergic component. Whereas, on inhibitory immunoblotting of TP-specific IgE, 5 micrograms/ml of DP extract completely inhibited all the IgE binding components of TP. These results suggested that the major allergen of TP may be the 18 kD component and we also concluded that TP allergens have a strong cross-reactivity with DP extracts, but that DP allergens only have partial cross-reactivity with TP extracts.