WE have previously shown that the activity of guinea pig γ-globulins as mediators of increased capillary permeability is markedly reduced by soya bean trypsin inhibitor (SBTI) and by di-isopropylphosphofluoridate (DFP)1. Because DFP was inhibitory, we tested esterase activity of guinea pig γ-globulins against synthetic amino-acid esters, using the Roberts2 modification of the Hestrin method. Other workers3 have shown that the enzyme kallikrein isolated from either human urine or plasma hydrolyses esters of arginine (for example, tosylarginine methyl ester, TAMe) specifically, whereas kallikrein isolated from hog pancreas will hydrolyse arginine esters and esters of tyrosine and tryptophan. We found that guinea pig γ-globulins would hydrolyse TAMe and to a smaller extent esters of lysine (for example, lysine methyl ester, LMe) but not esters of tyrosine, tryptophan, leucine or alanine. Furthermore, for four different batches of γ-globulins there appeared to be a relationship between permeability and esterase activities. Permeability activity was measured by intradermal injection into ‘blued’ guinea pigs as described previously1,4.