The sea urchin, Heliocidaris erythrogramma, has potent antibacterial activity in its coelomic fluid. Antibacterial activity was detected by a colorimetric bactericidal assay, which revealed a minimum inhibitory concentration of 680 ppm coelomic fluid v/v in FSW. The antibacterial activity of a number of molecules was induced by bacterial challenge with maximal antibacterial activity occurring after 12 h. The molecules responsible for antibacterial activity were heat-stable and also induced by non-specific challenge. Polar extracts of coelomic fluid indicated that at least one of the factors contributing to antibacterial activity was proteinaceous. The antimicrobial peptide was approximately 13 9 kDa as determined by SDS-PAGE. The amino acid sequence and potency of the antimicrobial peptide is also discussed.