The environment contains a huge array of allergens, primarily proteins or glycoproteins, to which the atopic individual may become sensitized. In situations of high exposure and highly reactive substances, even nonatopic individuals may develop allergic antibodies. The individual's response depends on the underlying immune responsiveness, the dosage, frequency, route, and possibly the chemical nature of the allergen. Most allergenic sources contain multiple allergenic determinants, as demonstrated by characterization of pollen and mold extracts. Major allergens are acidic proteins with weights in the 20,000 to 40,000 dalton range, and pIs in the 4 to 6 range. Minor allergens may be larger or smaller and either acidic or basic. These conclusions are based on the isolation of a relatively small number of allergens. Cross-reactivity data, especially with the pollens, are useful in the rational formulation of immunotherapy extracts. Considering the wealth of allergenic pollens, information available to date is very scanty. Data on molds, epidermals, and mites are even more preliminary or conflicting. As further research proceeds on purifying and characterizing allergens, a number of these questions and conflicts will be resolved.