The immunomodulatory effects of a hot water extract from matured fruit of the date palm tree (Phoenix dactylifera L.) were investigated in comparison to those of prune and fig fruit in mice. The number of spleen IFN-γ(+)CD4(+), IFN-γ(+)CD49b(+) and IL-12(+)CD11b(+) cells was highest in mice given the date extract-added diet. Polyphenols identified in the date extract, such as chlorogenic acid, caffeic acid, pelargonin and ferulic acid, stimulated IFN-γ mRNA expression significantly in mouse Peyer's patch cell cultures. Chlorogenic acid and caffeic acid also increased the number of IFN-γ(+)CD4(+) cells significantly, while some polyphenols increased the number of IFN-γ(+)CD49b(+) and IL-12(+)CD11b(+) cells significantly. On the other hand, a 70% ethanol-insoluble date extract treated with trypsin increased the number of IFN-γ(+)CD49b(+) and IL-12(+)CD11b(+) cells significantly. These results indicate that some polyphenols and polysaccharides present in date fruit stimulate the cellular immune system in mice.