Rice bran, a by-product of the rice dehulling process, contains bioactive polysaccharides which have immunomodulatory activity. We prepared fermented rice bran with Lentinus edodes mycelia [Erom's fermented rice bran (EFR)] to extract the polysaccharides and improve the immune-enhancing properties. The objective of this study was to evaluate the in vitro and ex vivo immunostimulatory effects of EFR on mouse macrophages and splenocytes. We investigated the effects of EFR on the production of nitric oxide (NO) and cytokines using the Griess assay and enzyme linked immunosorbent assay (ELISA), while the expression of inducible nitric oxide (iNOS) and cytokines in RAW264.7 cells was measured using reverse transcription-polymerase chain reaction (RT-PCR), Additionally, peritoneal macrophages and splenocytes were isolated from C57/BL6 mice. The macrophage lysosomal enzyme activity and splenocyte proliferation were determined using a lysosomal acid phosphatase assay and 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assay, respectively. In vitro studies showed that EFR significantly increased the production of NO and cytokines [interleukin (IL)-6, IL-1β, IL-10, and tumor necrosis factor-α], and up-regulated the mRNA expression of iNOS and cytokines in RAW264.7 cells. Ex vivo studies revealed that EFR increased splenocyte proliferation and lysosomal enzyme activity of peritoneal macrophages. These findings indicate that EFR could enhance immunity by inducing macrophage activation and splenocyte proliferation. Therefore, EFR has potential as a source material for functional foods that boost the immune system.