Ribosome inactivating proteins (RIPs) are enzymes that inactivate ribosomes by eliminating one or more adenosine residues from rRNA, a 9,567-Da RIP with a novel N-terminal sequence was isolated from fresh fruiting bodies of the mushroom Hypsizigus marmoreus. The protein was unadsorbed on DEAE-cellulose, adsorbed on Affi-gel blue gel, and appeared as a single peak upon gel filtration on Superdex 75. The protein, designated as marmorin, inhibited proliferation of hepatoma Hep G2 cells and breast cancer MCF-7 cells, HIV-1 reverse transcriptase activity, and translation in the rabbit reticulocyte lysate system with an IC50 of 0.15 mu M, 5 mu M, 30 mu M, and 0.7 nM, respectively. Compared to RIPs from hairy gourd, bitter gourd, ridge gourd, garden pea, and the mushroom Flammulina velutipes, marmorin was more potent in its antiproliferative activity toward hepatoma (HepG2) and breast cancer (MCF-7) cells, similar in inhibitory potency toward HIV-1 reverse transcriptase (with the exception that it was more potent than ridge gourd RIP and bitter gourd RIP), and less potent in translation-inhibitory potency. Marmorin was devoid of antifungal, protease, RNase, mitogenic, anti-mitogenic, nitric oxide-inducing, hemagglutinating, and trypsin inhibitory activities.