The relationship between the Ig[immunoglobulin]E-mediated immunogenic response to Ascaris and similar responses to common inhaled allergens was determined. Of 170 allergic asthmatics from an area in which Ascaris infestation was endemic 66 had positive skin-prick reactions to Ascaris antigen. The number and size of skin-prick reactions to common inhaled allergens were greater in this group than in the 104 Ascaris-negative allergic asthmatics. Specific serum IgE antibodies were determined by the radio-allergosorbent test. The serum IgE levels for Ascaris and 3 common inhaled allergens correlated well with the skin reactions induced by the same antigens. Total IgE and serum IgE antibody levels for the 3 inhaled allergens were higher in patients who had positive results on skin testing with Ascaris antigen than in those with negative results. There was no similar association between Dermatophagoides pteronyssinus and 6 common inhaled allergens. An Ascaris-induced immune response was associated with enhanced IgE-mediated reactivity to common inhaled allergens in both allergic and clinically non-allergic subjects.