A new cytotoxic factor has been purified by anion exchange chromatography and gel filtration from the marine sponge Pachymatisma johnstonii and was named pachymatismin. The pure fraction contained 30% sugar, suggesting a glycoprotein. The apparent molecular mass of the isolated substance was 46 kDa on Sephacryl S-200 HR column. It gave a single band on polyacrylamide gel (SDS-PAGE). The glycoprotein had a IC 5 0 at 0.8-2 μg protein/ml and inhibited proliferation of all tumor cells tested. Among them, the cells of human non-small-cell-bronchopulmonary carcinoma line (NSCLC-N6) were used as an experimental model to search for the mechanism of action at the cell-cycle level. Cell growth appeared to be inhibited at the G 0 /G 1 phase of the cell cycle.