Peptidylarginine deiminase (PAD) catalyzes the conversion of arginyl residues in proteins to citrullyl residues in the presence of Ca2+. Recently, we obtained a monoclonal antibody, EH7, which reacted only with mouse PAD type II. Here, we describe immunohistochemical findings on the cellular localization of PAD type II in mouse fetus by using the monoclonal antibody. PAD type II is expressed in yolk-sac erythroid cells and the level of the enzyme in these cells decreases as the cells differentiate.