An organic extract of the marine diatom Skeletonema costatum was studied in vitro for its effect on asynchronous cells of a human non-small-cell bronchoplumanory carcinoma line (NSCLC-N6). Cell growth appeared to be inhibited in the G1 phase of the cell cycle, and kinetic studies in pretreated cells showed that this growth arrest was irreversible. These events are related to a terminal maturation induction.