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 11kDa in size and constituted the VL fragment of HIR. The K m and k c a t values of Chromozym TRY hydrolysis for HIR were 1.5x10 - 4 M and 6.2min - 1 , and for the VL fragment 7.3x10 - 4 M and 4.8x10 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.