The Arg-specific gingipains ofPorphyromonas gingivalisRgpA and RgpB have 97% identical sequences in their catalytic domains yet their propeptides are only 76% identical. RgpA isolates as a proteinase–adhesin complex (HRgpA) which hinders direct kinetic comparison of RgpAcatas a monomer with monomeric RgpB. We tested modifications ofrgpAidentifying a variant that enabled us to isolate histidine-tagged monomeric RgpA (rRgpAH). Kinetic comparisons between rRgpAH and RgpB used benzoyl-l-Arg-4-nitroanilide with and without cysteine and glycylglycine acceptor molecules. With no glycylglycine, values ofKm,Vmax,kcatandkcat/Kmfor each enzyme were similar, but with glycylglycineKmdecreased,Vmaxincreased andkcatincreased ~ twofold for RgpB but ~ sixfold for rRgpAH. Thekcat/Kmfor rRgpAH was unchanged whereas that of RgpB more than halved. Recombinant RgpA propeptide inhibited rRgpAH and RgpB withKi13 nM and 15 nMKirespectively slightly more effectively than RgpB propeptide which inhibited rRgpAH and RgpB withKi22 nM and 29 nM respectively (p< 0.0001); a result that may be attributable to the divergent propeptide sequences. Overall, the data for rRgpAH reflected observations previously made by others using HRgpA, indicating rRgpAH fidelity and confirming the first production and isolation of functional affinity tagged RgpA.