Abstract An immunoglobulin L chain (HIR) was treated with lysyl-endopeptidase. Gel filtration chromatography of the digestion mix identified a peak displaying a significantly higher specific catalytic activity than that of the original sample. The protein in the peak was 11 kDa in size and constituted the VL fragment of HIR. The K m and k cat values of Chromozym TRY hydrolysis for HIR were 1.5 × 10 −4 M and 6.2 min −1, and for the VL fragment 7.3 × 10 −4 M and 4.8 × 10 2 min −1, respectively. Three out of the five BJPs studied in this paper displayed elevated catalytic activity after processing with lysyl-endopeptidase. Similar results were also obtained for the complete antibody.