Fibroblast-type interstitial collagenase (E.C. 3.4.24.7) was associated with loosening of total hip prostheses in eight patients: there were four cemented stems and one cementless stem with the common type of loosening and two cemented stems and one cementless acetabular component with aggressive granulomatous lesions. The authors used a specific, well-characterized, heterologous, affinity-purified, polyclonal rabbit anti-human fibroblast collagenase antiserum applied in avidin-biotin-peroxidase-complex (ABC) staining. In the aggressive granulomatous type of loosening, collagenase was found in most of the fibroblast- and macrophagelike cells, including multinuclear giant cells and epithelioid cells in periprosthetic tissue. Collagenase-positive cells also were found in the periprosthetic tissue associated with common loosening. Collagenase was also found in capillary and postcapillary venule endothelial cells in the richly vascularized aggressive granulomatous tissue. Collagenase was extracted directly from the tissue samples and incubated with soluble Type I collagen. Collagen degradation products then were analyzed by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, and the three-fourths length degradation product quantitated by gel scanning densitometry. In both aggressive granulomatosis and the common type of loosening, extractable collagenase was found in tissue. No significant differences between the sample groups were detected in respect to total measurable collagenase, however. The extractable collagenase was present in a latent form that could be activated by the organomercurial procollagenase activator, phenylmercuric chloride (PMC). It is likely that interstitial collagenase contributes to rapid growth of reactive infiltrative tissue, loosening of the prosthesis associated with aggressive granulomatosis, and the periprosthetic lytic process associated with the common type of hip prosthesis loosening.