Although chemotherapy is one of several therapeutic options for cancer patients, the outcomes have been less satisfactory with palpable side effects in many cases. To help improve the efficacy of chemotherapy, we examined whether D-fraction (DF), a bioactive mushroom extract, would potentiate anticancer effects of those drugs in vitro. Seven types of cancer cell lines, including breast, lung, stomach, colon, prostate, bladder, and kidney cancers, were tested with various chemotherapeutic drugs and their combinations with DF. Compared to individual drugs alone, combinations of those drugs and DF have indeed improved anticancer activity, resulting in the significant (p < 0.05) cell viability reduction in all cancer cells. As vitamin C (VC) has been insinuated to potentiate bioactivity of DF, combination of DF (300 μg/ml) and VC (200 μM) was also examined. Such studies showed that DF/VC combination led to the drastic (≥90%) viability reductions in 6 cancer cells and the 70% reduction in 1 cancer cell type. These results were superior to those from combinations of drugs and DF. To assess if such a profound cell viability reduction by DF/VC combination might be attributed to apoptosis, this possibility was tested by Western blots. Analyses revealed that anti-apoptotic bcl-2 was down-regulated while pro-apoptotic Bax was up-regulated with DF/VC combination, indicating induction of apoptosis. It is plausible that DF/VC combination may induce apoptosis, resulting in the severe cell viability reduction. Therefore, DF may have a great potential as an adjuvant agent that can be combined with chemotherapeutic drugs or with VC for the improved treatment of various human malignancies.