The prepn., from culture filtrates of C. welchii type A, of a proteolytic enzyme, ''collagenase'' (k-toxin), which appears to attack only collagen and gelatin, is described. It has been obtained substantially free from the a-and [theta]-toxins of C. welchii and purified (with respect to non-dialyzable N) about 200-fold. The partially purified enzyme is optimally active over the pH range 6-7.5, and is stable in soln. in borate but not in phosphate buffers. An increase in activity and antibody-combining power after dialysis against borate buffer at pH 10.2 was observed. Sera neutralizing the activity of collagenase-containing filtrates against collagen do not inhibit their activity against gelatin. The elec-trophoretic properties of partially purified prepns. are discussed.