A water-soluble glucan (RCP-1) was prepared from the roots of Rubus crataegifolius Bge. by extraction with hot-water, deproteination by Sevag reagent, alpha-amylase treatment and ultrafiltration. RCP-1 consisted of only glucose, and its molecular weight was determined to be approximately 7KD by high performance gel permeation chromatography (HPGPC). Fourier transform infra-red spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (NMR), methylation and periodate oxidation analyses indicated that RCP-1 was an alpha-d-glucan. Its main chains were composed of (1-->4)- and (1-->6)-linked alpha-glucopyranosyls, and side chains were single alpha-glucopyranosyl residues attached to the O-6 of glucosyls in the main chains. RCP-1 could increase both cytotoxic activity against B16 melanoma cells and the production of nitric oxide (NO) of macrophages in vitro. Furthermore, in vivo bioassay tests indicated that RCP-1 could remarkably enhance T and B lymphocyte proliferations, augment the phagocytosis of macrophages and increase the tumour necrosis factor-alpha (TNF-alpha) levels in serum.