A substantial part of harbor sediments consists of silty material often contaminated with persistent compounds such as tributyltin, mineral oil, polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and trace metals at levels with ecological risk ranges from low to high. Laboratory bioassays can improve the hazard assessment of sediments in this intermediatie zone of contamination and make it easier to decide whether harbor sediments need remedial action or if dredged material may be disposed of in open waters.