Aeroallergens of the southeastern Missouri region were determined by skin test frequencies of 817 consecutive individuals and compared and contrasted with five years of air sampling using the Rotorod sampler. Household inhalants were found to induce the most frequently positive skin tests, especially house dust, feathers, house dust mite, wool, cat epithelium, tobacco, dog epithelium, and horse epithelium. Among the trees, pollen extracts ofAcer saccharum(maple),Acer negundo(box-elder),Carya ovata/C. illinoensis(hickory/pecan),Juglans nigra(walnut), andUlmus americana(elm) were the most commonly positive. A slightly higher level of skin reactivity was found to grass pollen extracts withAnlhoxanthum odoralum(sweet vernal),Phleum pratense(timothy),Cynodon dactylon(Bermuda),Agrostis alba(red top) andFestuca elatior(meadow fescue), all being quite similar in incidence, butSorghum halepense(Johnson grass) was clearly less important.Ambrosiaspp. (ragweed) andXanthium commune(cocklebur) were the most frequent weed reactions with ragweed the most prominent allergen of outside environmental origin. Of the genera of fungi, skin tests toEpicoccumwere most frequently positive. When reactivity level is considered, the other important fungi areAlternaria, Candida, Cladosporium, Curvularia, andFusarium. Air sampling data generally correlates well with the positive skin tests frequencies i.e., the most numerous pollen, ragweed, has the highest skin test frequency and reactivity level. Similarly among the fungi the most numerous spores,Cladosporium, Alternaria, Fusarium, andEpicoccumhave the highest frequencies and reactivity levels. Certain species appear to be able to sensitize patients in lower concentrations, such as box-elder and the grasses. Oak pollen andCladosporiumspores appear in large numbers, but seem less sensitizing.