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A novel secondary sponge metabolite, peloruside A (peloruside), isolated from the marine sponge Mycale sp. (New Zealand), was tested for its cytotoxic effects on mammalian cells in culture. The macrolide structure of peloruside is similar to that of the protein kinase C (PKC) activator, bryostatin‐1 (bryostatin), both containing a pyranose ring adjacent to a gemdimethyl moiety. Peloruside is a potent inhibitor of cell proliferation. Treatment of different mammalian cell lines with peloruside for 48–96 h gave IC50 values ranging from 4 to 15 nM, using the colorimetric 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5 diphenyl tetrazolium (MTT) cell proliferation assay. Peloruside was shown to be both cytostatic and cytotoxic by trypan blue dye exclusion tests. Peloruside induced apoptosis in a dose‐dependent manner in murine (32D) and human (HL‐60) myeloid cell lines, revealed by DNA laddering in agarose gels and flow cytometric …