A needle cylinder of a circular knitting machine includes a cylinder jacket having axial needle channels, knitting needles movable therein and an upper cylinder end for stitch formation having a radial groove between pairs of adjacent needle channels each having an insert forming a stitch formation surface with its insert surface protruding from the groove. Exchangeable, stitch formation surfaces form quickly replaceable inserts. Each insert is a spring lug including a lug foot overlapping the upper cylinder end at the side of the cylinder jacket or the side edge of an outer ring channel in the upper cylinder end and a lug terminal with a terminal part springily engaging behind a counter support projecting at the upper cylinder end at the cylinder's inner side or a side edge of an inner ring channel in the upper cylinder end. A circular knitting machine with the needle cylinder is also provided.