Peptidylarginine deiminase, which catalyzes the deimination of arginyl residues in protein, was identified and partially purified from rabbit skeletal muscle. The specific activity of the crude extract from the muscle was aboutt 120 times that of the extract from newborn rat epidermis. The enzyme was purified abou 250-fold over the initial extract of rabbit skeletal muscle in a yield of about 53%. The enzyme requires Ca2+ as an essential cofactor and the activity was not inhibited by monoiodoacetate or PCMB. The molecular weight of the enzyme was found to be about 115,000 by gel chromatography on Bio-Gel P-300. The enzyme catalyzed the deimination of arginyl residue when both the alpha-amino and alpha-carboxyl groups were substituted, and showed low activity when substrates had either a free alpha-amino or a free alpha-carboxyl group. The action of the enzyme on free L-arginine was negligible. Proteins such as protamine, histone, and ribonuclease A were better substrates than the other small synthetic peptides tested.