In search of natural bioactive microbial compounds with adjuvant properties, we have previously showed that the polysaccharopeptide (PSP), isolated from Chinese medicinal mushroom Coriolus versicolor, was able to enhance the cytotoxicity of certain S-phase targeted-drugs on human leukemic HL-60 cells via some cell-cycle and apoptotic-dependent pathways. The present study aimed to investigate whether the synergism of mechanisms of PSP with certain chemotherapeutic drugs also applies to human breast cancer. PSP treatment enhanced the cytotoxicity of doxorubicin (Doxo), etoposide (VP-16) but not cytarabine (Ara-C). Bivariate bromodeoxyuridine (BrdUrd)/DNA flow cytometry analysis estimated a longer DNA synthesis time (Ts) for the PSP treated cancerous cells suggesting that PSP enhanced the apoptotic effect of Doxo and VP-16 via creating an S-phase trap in the human breast cancer cell line ZR-75-30. The participation of PSP in the apoptotic machinery of the chemotherapeutic agents was further supported by a reduced ratio of protein expression of Bcl-xL/Bax of the cancer cells. This study provides further insight into the synergistic mechanisms of PSP and supports the hypothesis that the anticancer potentials of PSP is not limited to leukemia but may also be used as an adjuvant therapy for breast cancers.