Objectives: Marine organisms have proven to be a rich source of potent pharmacologically active compounds. Three polyprenyl-1,4-hydroquinone derivates (hexaprenyl-1,4-hydroquinone, heptaprenyl-1,4-hydroquinone and nonaprenyl-1,4-hydroquinone) were isolated from the Zoobenthos-inhabiting sponges Sarcotragus muscarum and Ircinia fasciculata from the Eastern Mediterranean Sea (phylum: Porifera; class: Demospongiae). Methods: Hexa-, hepta- and nonaprenylhydroquinone were identified by 1H-NMR, H,H-COSY, heteronuclear multiple bond correlation, FAB-MS and UV spectroscopy. The effects of the compounds on cell viability was determined using the MTT assay; antioxidative potential was measured using the Trolox equivalent antioxidative capacity assay. Inhibition of nuclear factor-κB activity was detected by secreted alkaline phosphatase assay. Activity against an array of protein kinases was determined in 96-well FlashPlates. Key findings: All compounds had prominent antioxidative activity, comparable to that of the synthetic vitamin E derivate Trolox. Hexaprenylhydroquinone showed the greatest cytotoxicity in H4IIE hepatoma cells (EC50 2.5 μM). All three compounds inhibited NF-κB signalling in this cell line, with heptaprenylhydroquinone being the most active. Screening of 23 kinases involved in signal transduction pathways (cell proliferation, survival, angiogenesis and metastasis) showed that hexaprenylhydroquinone and heptaprenylhydroquinone inhibited the activity of the epidermal growth factor receptor (IC50 1.6 and 1.4 μg/ml, respectively), and heptaprenylhydroquinone also inhibited the activity of other kinases (Src tyrosine kinase, vascular endothelial growth factor receptor 3 and insulin-like growth factor 1 receptor). Conclusions: The prenylated hydroquinones isolated from the marine sponges S. muscarum and I. fasciculata showed cytotoxic and antioxidative activities and inhibited NF-κB signalling in H4IIE hepatoma cells and protein kinases. These findings may result in the generation of new lead substances in cancer therapy. © 2009 The Authors.