Objective. To determine if circulating gelatinase activity and matrix metalloproteinase 9 (MMP-9) (gelatinase B, or 92-kd type IV collagenase) antigenic levels are elevated in sera of patients with giant cell arteritis (GCA), and to ascertain if MMP-9 messenger RNA (mRNA) is deposited in situ at sites of disease involvement.Methods. Serum samples were collected from 12 patients with GCA and 12 healthy volunteers. Vascular tissue was obtained a t the time of temporal artery biopsy. Type IV collagenase activity was determined by gelatin substrate zymography and the quantitative biotinylated gelatin substrate degradation assay. A doublesandwich immunoassay utilizing 2 different isotypes of monoclonal antibodies generated against MMP-9 was used for measuring serum MMP-9 antigenic levels. Finally, to localize sites of MMP-9 mRNA transcription in inflamed arteries, the method of reverse transcriptase in situ polymerase chain reaction (RTisPCR) was utilized.Results. Serum gelatinase activity and MMP-9 titers were significantly increased in patients with GCA (mean f SEM 198.9 f 36.9 p g gelatin/hour/ml serum, versus 21.2 f 4.0 in controls; P = 0.0006). The differences in antigenic MMP-9 levels were even more prominent (3005.4 & 900.6 ng/ml and 31.6 -C 9.8 ng/ml in GCA aIid control sera, respectively; P = 0.007). By