Summary: Rhesus monkeys sensitized by intradermal inoculations with either Ascaris suum extracts or an electrophoretically purified fraction developed wheal and flare reactions upon intradermal challenge. Persistence of the skin reactivity in the sensitized animals lasted between 40 and 90 days, and all animals responded to restimulation with Ascaris extracts and attained their previous level of sensitivity. None of the sensitized animals showed any sign of either Arthus or delayed hypersensitivity reactions. Attempts to demonstrate circulating anti-Ascaris antibody by PCA or precipitin (ring) tests were unsuccessful. However, the presence of homologous skin-fixing antibody in the sera of Ascaris-sensitized rhesus monkeys was demonstrated by in vitro histamine release from passively sensitized monkey lung tissue. These studies indicate that crude Ascaris extract contains two components, separated by electrophoresis, with distinct biologic properties. The cathode migrating fraction 1) is nontoxic in guinea pigs, 2) induces wheal and flare reactions in normal animals, 3) elicits wheal and flare reactions in sensitized animals and 4) releases histamine from passively sensitized monkey lung tissue. The anode migrating fraction is toxic in guinea pigs and is unable to 1) sensitize normal animals, 2) elicit wheal and flare reactions in sensitized animals and 3) release histamine from passively sensitized lung tissue.