The incidence and enzymatic activity of penicillin‐degrading enzymes (beta‐lactamases) in gingival crevicular fluid were determined for 406 sites in 52 patients with adult periodonititis. Beta‐lactamase activity was detected in 64 of the patients and in 24 of the sites sampled. The enzyme was present in 13 of 206 sites and 36 of 200 sites having probing depths of ≤3 mm and>3 mm, respectively. The incidence of the enzyme was positively correlated with increases in pocket depth up to 8 mm. However, there was no correlation between probing depths and the enzyme activity present. The range of activity detected was equivalent to 0.001 to 0.569 enzyme units per ml of gingival fluid. Approximately 60 of the sites in which beta‐lactamase was detected, or 19 of all sites sampled with probing depth>3 mm, contained enzymatic activity sufficent to inactivate penicillins at the concentrations available in gingi
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