While IgE and IgG antibodies are thought to play important pathogenetic roles in allergic bronchopulmonary aspergillosis (ABA), the microscopic lung picture would implicate cell-mediated immunity. Patients with ABA do not manifest delayed skin test reaction to Aspergillus fumigatus (AF). This has been attributed to inhibition of lymphokines by histamine locally released as a result of the immediate skin test reaction. We tested 5 ABA patients for delayed hypersensitivity to AF. An attempt was made to ablate the immediate skin test reaction to determine if a delayed reaction might be unmasked. At concentrations of AF previously shown to produce marked immediate skin test reactions, the combination of cimetidine by mouth and chlorpheniramine mixed with AF in the skin test syringe completely eliminated the immediate skin test reaction in all 5 patients. In no case did a delayed reaction appear. Eight ABA patients showed in vitro lymphocyte stimulation responses to AF that were comparable to those of asthmatics and rhinitics with positive immediate skin test reactions to AF and no clinical evidence of ABA. It thus appears that 'peripheral' cell-mediated immune responses, namely delayed skin reactivity and peripheral blood lymphocyte proliferation to AF, are not present in ABA.