Peanuts are one of the most allergenic foods. The allergic reactions may vary in severity from mild urticaria to severe anaphylactic episodes and death. The prevalence of peanut sensitivity is unknown, but it may affect as many as 10% of allergic individuals. The chemistry of peanut proteins has been extensively studied. Two major protein fractions have been prepared from saline extracts of peanut flour, arachin and conarachin. A major peanut allergen termed "Peanut-1" has been isolated. However, a number of protein constituents, including the arachin and conarachin fractions, have been shown to be allergenic. The ability to diagnose peanut sensitivity accurately has been hampered by the lack of standardized peanut extracts. However, efforts are under way to prepare such standardized reagents. Treatment consists of avoiding peanut protein products and using self-administered epinephrine. A number of peanut protein-containing products are allergenic, although peanut oil is not. The peanut-allergic consumer should be instructed to carefully read labels of foods. This can at times, however, be misleading, because certain foods may be inadvertently contaminated by peanut proteins.