: The effect of a marine bicyclic peptide, theonellamide F, on vacuole formation in exponentially growing 3Y1 rat embryonic fibroblasts was studied in comparison with the effect of monensin. Many abnormally large vacuoles appeared around the nucleus in the cells treated with 6 µM theonellamide F within 24 hours. Following prolonged treatment with this agent, the number of enlarged vacuoles increased. Such vacuoles accumulated many granules that showed Brownian movement. The effect of theonellamide F on the cells was more drastic in an amino-acid-deficient medium, in which all cells died within 1 hour at a 3-µM concentration of the agent. Theonellamide F probably affects cellular autophagy, inhibiting the degradation of the organelles and turnover of proteins. Monensin, a well-known Na(+) ionophore that disintegrates the Golgi apparatus, induced similar types of vacuole formation, although the vacuoles were localized in a region slightly distant from the nucleus. Monensin readily affected cell morphology, resulting in cell death. We propose that theonellamide F, like monensin, is a useful agent for investigating membrane structures in cells.