Rabbit alveolar macrophages were cultured in an environment conducive to the secretion of both reactive oxygen and proteinases, so that the relative importance of proteolytic and oxidative inactivation of alpha 1-proteinase inhibitor by alveolar macrophages could be evaluated. The inactivation of alpha 1-proteinase inhibitor was proportional to its proteolysis, and there was no detectable inactivation in the absence of proteolysis. Although the live macrophages were capable of secreting reactive oxygen, they did not inactivate alpha 1-proteinase inhibitor by oxidation. The inactivation of alpha 1-proteinase inhibitor by proteolysis was proportional to the secretion of elastinolytic activity by the alveolar macrophages. The inability of the alveolar macrophages to oxidize alpha 1-proteinase inhibitor was attributed to the methionine in the macrophages, in secreted proteins, and in the culture medium competing for oxidants. The data suggest that proteolytic inactivation of alpha 1-proteinase inhibitor may be important in vivo and that the methionine concentration in vivo may protect alpha 1-proteinase inhibitor from significant oxidative inactivation.