An immunomodulatory polysaccharide-rich substance (Noni-ppt) from the fruit juice of Morinda citrifolia has been found to possess both prophylactic and therapeutic potentials against the immunomodulator sensitive Sarcoma 180 tumour system. The antitumour activity of Noni-ppt produced a cure rate of 25%–45% in allogeneic mice and its activity was completely abolished by the concomitant administration of speciﬁc inhibitors of macrophages (2-chloroadenosine), T cells (cyclosporine) or natural killer (NK) cells (anti-asialo GM1 antibody). Noni-ppt showed synergistic or additive beneﬁcial effects when combined with a broad spectrum of chemotherapeutic drugs, including cisplatin, adriamycin, mitomycin-C, bleomycin, etoposide, 5-ﬂuorouracil, vincristine or camptothecin. It was not beneﬁcial when combined with paclitaxel, cytosine arabinoside, or immunosuppressive anticancer drugs such as cyclophosphamide, methotrexate or 6-thioguanine. Nonippt also demonstrated beneﬁcial effects when combined with the Th1 cytokine, interferon gamma, but its activity was abolished when combined with Th2 cytokines, interleukin-4 or interleukin-10, thereby suggesting that Noni-ppt induces a Th1 dominant immune status in vivo. The combination of Noni-ppt with imexon, a synthetic immunomodulator, also demonstrated beneﬁcial effects, but not when combined with the MVE-2 copolymer, a high molecular weight immunomodulator. It was also not effective when combined with interleukin-2 or interleukin-12.