<jats:title>Abstract</jats:title><jats:p>Five guanidine alkaloids, mirabilin B (<jats:bold>1</jats:bold>), 8b<jats:italic>β</jats:italic>‐hydroxyptilocaulin (<jats:bold>2</jats:bold>), ptilocaulin (<jats:bold>3</jats:bold>), and a mixture of the 8<jats:italic>β</jats:italic>‐ and 8<jats:italic>α</jats:italic>‐epimers, <jats:bold>4</jats:bold> and <jats:bold>5</jats:bold>, of 8‐hydroxymirabilin (1,8a;8b,3a‐didehydro‐8‐hydroxyptilocaulin), were isolated from <jats:italic>Monanchora arbuscula</jats:italic> colonies collected off the northeastern Brazilian coast. All structures were elucidated by spectroscopic analysis, including 1D (<jats:sup>1</jats:sup>H‐, <jats:sup>13</jats:sup>C‐ (BB), and <jats:sup>13</jats:sup>C‐DEPT) and 2D (COSY, HSQC, and HMBC) NMR experiments, and comparison with the literature data. The cytotoxicity of the isolated compounds were evaluated against four tumor cell lines, showing that mirabilin B (<jats:bold>1</jats:bold>) and the two epimers were inactive, while 8b<jats:italic>β</jats:italic>‐hydroxyptilocaulin (<jats:bold>2</jats:bold>) and ptilocaulin (<jats:bold>3</jats:bold>) presented <jats:italic>IC</jats:italic><jats:sub>50</jats:sub> values in the range of 7.9 to 61.5 μ<jats:sc>M</jats:sc>, and 5.8 to 40.0 μ<jats:sc>M</jats:sc>, respectively. Further studies on the mechanism of action of ptilocaulin, using HL‐60 leukemia cells, demonstrated that this guanidine compound induced apoptosis of the treated cells.