A marine bacterium D2 (CCUG 26757) isolated from a tunicateCiona intestinalisspecimen produced a low molecular weight component which inhibits barnacle and tunicate larvae and prevents their settlement on solid surfaces (Holmströmet al., 1992). In order to perform chemical and structural analyses of the component independent of season, a bioassay, complementary to tests with invertebrate larvae was developed. This bioassay is based on tissue cell culture techniques and growth of the AGS cell line. It was previously shown that the toxic component is a stationary phase released product, which is heat stable and less than 500 Dalton in size (Holmströmet al., 1992). Furthermore, it is not a peptide or a protein and metaperiodate treatment increases its toxicity to larvae indicating that it binds to or contains carbohydrate moieties. In this study, these results were confirmed by using the anchorage dependent human gastric adenocarcinoma (AGS) cell line as a bioassay. Fractionation of the D2 supernatant on a Sephadex G‐200 column and addition of different size fractions to the AGS tissue culture cells, showed that both a low and a high molecular weight fraction inhibited cell growth. Exposure ofBalanus amphitriteandC. intestinalislarvae to the same fractions, showed that the low molecular weight fraction that inhibited growth of the cells corresponds to the component that inhibited larvae of both organisms. The high molecular weight fraction was found to inhibit larvae ofB. amphitrite.