A highly active soluble peroxidase has been identified in the preputial glands of the rat. The enzyme was detectable in the sebaceous secretion of the glands and showed catalytic properties characteristic of true peroxidase. It had a native molecular weight of around 73 000 as determined by gel-permeation studies. Immunologically the enzyme cross-reacted with an antiserum against bovine lactoperoxidase. Administration of progesterone resulted in a significant increase in the total activity of the enzyme, while testosterone and oestradiol had no such effect. The enzyme had a similar molecular weight and similar catalytic and immunological properties to rat uterine fluid peroxidase but differed markedly in respect to sensitivity to oestradiol.