In previous studies with isolated perfused rabbit lungs, we observed that inhaled human serum albumin (HSA) or ovalbumin (OA) entered the pulmonary circulation antigenically intact. The inhaled proteins were also metabolized in the lung. Immunization reduced the amount of intact protein and increased the amount of metabolites absorbed. In the present study, we have begun to characterize the immune mechanisms responsible for reduced antigen absorption. A humoral immune mechanism appeared to be involved because the phenomenon could be passively transferred to normal animals by administering immune serum either 18 hr or immediately before antigen inhalation. The reduction was also observed when lungs from immunized rabbits were perfused with normal rabbit blood, indicating that antibodies in both lung and blood may be involved. Experiments, in which lungs from immunized rabbits were simultaneously insufflated with the immunizing antigen and with a nonspecific protein, demonstrated that the block against antigen absorption was specific for the immunizing antigen and was not due to some antigen-induced nonspecific changes in lung physiology.