A new formaldehyde-free wood-adhesive system consisting of kraft lignin and polyethylenimine (PEI) was investigated for making two-ply plywood. The lignin-PEI adhesive was prepared by mixing a kraft lignin solution and a PEI solution at room temperature. The mixing time of the lignin and PEI had little effect on the shear strength when it was >= 40 min. The shear strength of the resulting wood composites increased slightly as a function of increased hot-press time from 2 to 9 min. The optimum hot-press temperature was 140 degrees C. The lignin-PEI adhesive with a lignin/PEI weight ratio of 2: 1 resulted in the highest shear strength and the highest water resistance. The wood composites bonded with the lignin-PEI adhesives retained very high shear strengths after undergoing a water-soaking-and-drying test, as well as a boiling water test. Storage of the lignin-PEI adhesive for three days did not have detrimental effect on plywood shear strength. The molecular weight (M-w) of PEIs in the range of 70 000 to 150 000 had little effect on the shear strength of the resulting wood composites. The cure chemistry of this lignin-PEI adhesive was proposed to be similar to that of the quinone-tanning process.