Marine fish farms use many antibacterial agents to prevent or fight bacterial diseases. The major part of the orally supplied drugs reach the environment, either directly due to excessive feeding and reduced appetite of the cultured fish, or indirectly after passing through the fish. Some of the drugs entering the environment may be taken up by bivalves. Comparative contamination and decontamination of Japanese oyster Crassostrea gigas and blue mussel Mytilus edulis were studied when these bivalves were kept during 10 d in seawater containing oxytetracycline (OTC) or oxolinic acid (OA) at concentrations of 0.15 and 1.50 mg l(-1). After the 10 d exposure at the seawater concentrations of 0.15 and 1.50 mg l(-1), OTC and OA concentrations in bivalve tissues were respectively 0.09 to 0.16 and 0.70 to 1.40 mg kg(-1). OTC, which is in a large part bound to ions and organic molecules in seawater, was less absorbed by bivalves than OA and therefore contaminated their tissues less than did OA. However, OTC, whose binding to mineral and organic compounds of the bivalve tissues is stronger than OA, was eliminated more slowly from the bivalve tissues than OA. Differences of contamination and decontamination of the bivalve species may be related to some chemical properties of the antibacterial agents and to some anatomical and physiological specificities of the bivalve. The biotransformations of the antibacterials agents by the bivalves did not seem to play a prominent part in the contamination and decontamination of the bivalve tissues.