The concentration of doxycycline required to inhibit 50% (50% inhibitory concentration for serpinase activity) of alpha-1-antitrypsin degradation by purified neutrophil collagenase was found to be approximately 20 microM, a value similar to the 50% inhibitory concentration of doxycycline required to inhibit collagen degradation by neutrophil collagenase. Doxycycline also efficiently inhibited phorbol myristate acetate-triggered neutrophil-mediated degradation of alpha-1-antitrypsin. This suggests that doxycycline can protect alpha-1-antitrypsin from collagenase and gelatinase in the presence of other proteases and biologically active molecules that are released by triggered neutrophils. The protection of a body's alpha-1-antitrypsin shield from serpinolytic activity of collagenase and matrix metallproteinases can result in inhibition of serine proteases such as neutrophil elastase. Tetracyclines may thus protect matrix constituents from a wider spectrum of neutral proteases than previously recognized, not just from the matrix metalloproteinases collagenase and gelatinase.