Pachymatismin is a new cytostatic factor extracted from the marine sponge Pachymatisma johnstonii Bowerbank 1842 (Demospongiae, Geodiidae), which has an antiproliferative effect in vitro on cells from a human non-small-cell bronchopulmonary carcinoma (NSCLC-N6). The substance, both administered as a continuous and discontinuous treatment, triggers the irreversible arrest of cells in the G0/G1 phase of the cell cycle and morphological changes, thereby causing their destruction. Taken all together, these observations suggest that pachymatismin would induce atypical terminal cellular differentiation.