FMF indicates familial Mediterranean fever. C5a-inhibitor activity was measured as the percentage of inhibition of recombinant C5a (rC5a)–induced release of myeloperoxidase (MPO) from neutrophils as described. A 50-μL volume of serum-free culture medium (3rd through 5th passage) was incubated with 50 μL of 5 nmol/L rC5a for 20 minutes at 37°C before the MPO release assay. For each assay carried out with a putative inhibitor source, a control assay was carried out under conditions identical in every respect except that the inhibitor source was replaced by an equal volume of serum-free medium. For each FMF culture, a culture from the same normal tissue, at the same passage, was assayed. A₄₉₀ readings for control MPO release were 0.35 to 0.67. The results are expressed as the mean ± SE. The percentage of C5a inhibition produced by all the synovial and peritoneal normal cultures was highly significant (P < .001), whereas no significant inhibition was found in the normal skin or in FMF skin and peritoneal fibroblast cultures.