We studied the effect of leukotriene D4 (LTD4) given by aerosol challenge on asthmatic and normal monkeys. The response to LTD4 occurred within 2 min., was prolonged at higher concentrations, was reversed by epinephrine, and more closely resembled an antigen response than a histamine response. Since LTD4 concentrations of 30–100 μg/ml produced a response similar to 2–20 mg/ml of histamine, the LTD4 was 300–900 times as potent as histamine on a molar basis. Thus, LTD4, which appears to reproduce an antigen-induced response in rhesus monkeys, may be a mediator of allergic asthma in these animals.