Elongation factor 1α (EF-1α) is an essential factor for protein synthesis in eukaryotes. Here, we demonstrated that Tetrahymena EF-1α induced bundles of rabbit skeletal muscle F-actin as well as Tetrahymena F-actin in vitro, although Tetrahymena and skeletal muscle actins are different in some parts of their primary structures and in the binding abilities to some actin-binding proteins. Co-sedimentation experiments showed that the binding ratio of Tetrahymena EF-1α to skeletal muscle F-actin in the bundles was 1:1 . Electron microscopic observation showed that alkaline pH or high ionic strength reduced the bundling activity of Tetrahymena EF-1α to some extent, although the EF-1α seemed to be able to induce bundling of the F-actin within the range of physiological condition.