When knotted loops of suture material were subjected to increasing tensile stress it was the knot that gave way in 710 of 720 experiments. The knot is thus the weakest point in the loop. A new code for describing knots useful in surgery has been worked out. The tensile strength of 12 different knots using twelve different suture materials with the dimension 3/0 (USP) was examined systematically. It was found that the efficiency of the knots varied depending upon the tensile strength of the material, 5% for the weakest and 99% for the strongest knot-material combination. Monofil and multifil steel threads were superior to other materials examined with regard to the knot strength. Chromic and plain catgut showed at least 50% of the tensile strength of the corresponding thread, irrespective of the type of knot. Silk was among the materials which gave the weakest knots. In principle, the more turns and throws the stronger the knot.