An actin-modulating protein (AM-protein) isolated from the acellular slime mold Physarum polycephalum and microinjected into living Amoeba proteus causes characteristic changes in cell shape, locomotory behaviour, and organization of the microfilament system of the amebae. The peptide chain weight of the AM-protein, which binds to one actin molecule with high affinity thus forming a heterodimer, is 43 000. The heterodimer is a powerful inhibitor of actin polymerization, when added to G-actin. A 50% inhibition is already obtained at a weight ratio of 0.05 AM-protein/actin. The AM-protein heterodimer has only little effect on F-actin, which can be neglected under the experimental conditions used. The microinjected AM-protein transforms normal locomoting polytactic amebae into orthotactic cell models. The orthotactic forms are characterized by a polarized organization, i.e. a flattened advancing cell pole consisting of one large pseudopod showing clear suppression of further pseudopod formation, and a contracting uroid undergoing cellular budding processes by the formation and vesiculation of numerous small evaginations. The observed changes in morphology can be interpreted as a relaxation of the advancing cell region due to a significant reduction of the polymerizable actin pool. As the AM-protein heterodimer does not attack F-actin filaments or the interaction of actin and myosin the middle region and the uroid of the microinjected cells are able to perform contractions which result in an orthotactic moving pattern. In conjunction with control experiments using double injection of AM-protein and phalloidin our observations prove the important role of controlled actin polymerization and depolymerization processes for cytoplasmic streaming activity and cell locomotion in ameboid cells.