A monoclonal antibody (Pea-25) directed to phytochrome from etiolated peas (Pisum sativum L., cv Alaska) binds to an antigenic domain that has been highly conserved throughout evolution. Antigenic cross-reactivity was evaluated by immunoblotting sodium dodecyl sulfate sample buffer extracts prepared from lyophilized tissue samples or freshly harvested algae. Pea-25 immunostained an approximately 120-kilodalton polypeptide from a variety of etiolated and green plant tissues, including both monocotyledons and dicotyledons. Moreover, Pea-25 immunostained a similarly sized polypeptide from the moss Physcomitrella, and from the algae Mougeotia, Mesotaenium, and Chlamydomonas. Because Pea-25 is directed to phytochrome, and because it stains a polypeptide about the size of oat phytochrome, it is likely that Pea-25 is detecting phytochrome in each case. The conserved domain that is recognized by Pea-25 is on the nonchromophore bearing, carboxyl half of phytochrome from etiolated oats. Identification of this highly conserved antigenic domain creates the potential to expand investigations of phytochrome at a cellular and molecular level to organisms, such as Chlamydomonas, that offer unique experimental advantages.