A study of the coefficient of correlation has been made, based on cutaneous response and by intradermal means, between household dust from Barcelona diluted to a strength of 1/1,000 and an extract of D. Pteronyssinus and its medium of culture, diluted to a strength of 1/1,000,000. This study was effected to a group of thirty-three children with allergic asthma, sensitive to household dust, with a resultant value of r = 0.94 (p less than 0.001). In the same way, thirty-six children with the same characteristics, the correlations between the same extracts and an extract of dust without mites has been studied. Using Spiekma's technic, this dust was obtained through successive sifts gathered in a case under the inferior sifter. Of the mentioned dust, two equal fractions were made, one to confirm the absence of mites (Spiekma's technique) and the other to realize the extract, preparing the dilutions 1/1,000 and 1/1,000,000. With the dilution 1/1,000, the value is r = 1 (p less than 0.001) for the domestic dust and of r = 0.94 (p less than 0.001) for the D. Pteronyssinus. In sixteen of the above mentioned children, the study was repeated with the same extract, using the dilution of 1/1,000,000, and the values obtained were of r for household dust of 0.004 (p less than 0.55) and for D. Pteronyssinus of 0.016 (p less than 0.55). In twenty-one of these children, unmited dust and the extract of the way of cultivation (9 parts of human skin scales and 1 part of Sacharomyces) was compared, both diluted to 1/1,000. The value obtained was of r = 0.89 (p less than 0.001). Taking these results into consideration, it seems that D. Pteronyssinus is the most powerful allergene of dust, but the possible role of fecal pellets, product of mite excretion and of human skin scales should be considered as minor allergenes. The human skin scales emphasize their role in this case, and even though good correlation is obtained, we need to use concentrations a thousand times bigger than the D. Pteronyssinus. One can think of it being the same antigen, but the study made previously between uncultivated mites in human skin scales and these in concentrations of the same potence, has been of 0.31, which leads us to think that it is not the same antigene. Finally we consider that in some conditions, it is possible that other antigenes participate in the genesis of dust allergic (fecal pellets, human skin scales), but the antigenic power is very low.