The invention relates to a novel class of serine proteases of peptidase family S2A or S1E that are stable in the presence of copper (Cu2+) and/or inhibited by copper only to a limited extent. Structural features of potential relevance for this effect are also disclosed. This class of proteases includes proteases derived from Brachysporiella gayana, Nocardiopsis dassonvillei subsp. dassonvillei, Nocardiopsis prasina, and Nocardiopsis alba. The invention also relates to DNA encoding such proteases and the expression thereof in a host cell, animal and plant cells, as well as to the use thereof in animal feed and in detergents. In particular, the invention relates to animal feed and animal feed additives, such as premix, incorporating these proteases together with 1-500 ppm Cu (in-feed-concentration).