ABSTRACT A pea chloroplast protein resembles vertebrate and algal actins by several chemical and immunological criteria. On two-dimensional polyacrylamide gels it migrated with a slightly lower relative molecular mass (Mr = 41 000) and slightly more basic isoelectric point than rabbit skeletal muscle actin. A monoclonal antibody to chicken gizzard actin reacted on immunoblots with rabbit skeletal actin, with Chara actin and with a 41000Mr band from pea chloroplasts. Pea and Chara bands of approximately 58 000Mr were also stained. A DNase I-affinity column that bound muscle actin also bound 41 000 and 58 000Mr chloroplast polypeptides. Similarities existed between enzymically and chemically generated fragments of the 41000Mr chloroplast polypeptide and rabbit muscle actin. The 41000 Mr protein was protected from degradation by thermolysin only in preparations of intact, but not ruptured, isolated chloroplasts, indicating that this protein resides within the outer envelope membrane of these organelles. It is concluded that a 41 000 Mr protein with major similarities to actin occurs inside pea chloroplasts, and that a 58000Mr protein with some similarities to actin also probably exists within chloroplasts.