Girolline, an antitumor compound isolated from a sponge, has been reported to inhibit the termination step of protein synthesis in vivo. In this study, we found that girolline induced G2/M cell cycle arrest in several tumor cell lines. Immunochemical analysis revealed that polyubiquitinated p53 was accumulated in girolline-treated cells, while other polyubiquitinated cellular proteins were not accumulated, indicating that the effect of girolline is specific for p53. On the other hand, girolline did not inhibit proteasome activity in vitro, and accumulation of polyubiquitinated p53 was scarcely detected in the presence of leptomycin B, an inhibitor of nuclear export. Based on the above findings, we propose that girolline affects the step of recruitment of polyubiquitinated p53 to the proteasome.