The allergenic cross-reactivity of both inter- and intraspecies of house dust mites, Dermatophagoides pteronyssinus (Trouessart, 1897) and Dermatophagoides farinae Hughes, 1961, taking into account the allergenic differences that exist throughout the growth curves, was evaluated by means of RAST-inhibition, using sera from patients allergic to these mites. The results demonstrate that extracts obtained from mite cultures during the maximum exponential growth phase are the best source of reagents to better discriminate cross-reactivity studies. The analyses obtained from this work, together with those obtained in previous reports, help to define the ideal conditions related to the allergenic diversity, avidity, and cross-reactivity of specific antibodies for the elaboration of allergenic extracts as a tool for use in diagnosis and specific treatment of IgE-mediated hypersensitivity caused by house dust mites.