We have shown that a family of high-molecular-mass proteins can be detected in lysates of parasitized erythrocytes using antibodies specific for a Plasmodium yoelii rhoptry protein. When these polypeptides are biosynthetically labeled in the presence of brefeldin A or at 15 degrees C, their electrophoretic mobility on polyacrylamide gels is decreased. Removal of the drug restores the size of the polypeptides to that in the absence of the drug. These results indicate that the proteins undergo a processing event, most probably a proteolytic cleavage, which is inhibited by brefeldin A and low temperature. The data suggest that the proteins are moved by secretory transport from the endoplasmic reticulum through a functional Golgi to the rhoptry organelles.