We have investigated the protective effects of grape seed proanthocyanidins on doxorubicin-induced toxicity in tumour-bearing mice. The intraperitoneal administration of doxorubicin (2 mg kg(-1) every other day, cumulative dosage for 18 mg kg(-1)) significantly inhibited the growth of sarcoma 180, and induced myocardial oxidative stress with decreased superoxide dismutase and glutathione peroxidase activity while increasing malondialdehyde formation in the heart or serum. Doxorubicin-induced myocardial oxidative stress also reduced lactate dehydrogenase and creatine kinase activity in the heart and elevated their levels in the serum. Doxorubicin also affected immune functions of tumour-bearing mice with significantly decreased interleukin-2 (IL-2) and interferon-gamma (INF-gamma) production, and slightly decreased natural killer (NK) cell cytotoxicity, lymphocyte proliferation and CD4+/CD8+ ratio. It markedly increased the percentages of cytotoxic T cells (CD3+CD8+), helper T cells (CD3+CD4+), IL-2R+CD4+, and IL-2R+ cells as compared with untreated tumour-bearing mice. The intragastric administration of proanthocyanidin (200 mg kg(-1) daily) significantly inhibited tumour growth, and increased NK cell cytotoxicity, lymphocyte proliferation, CD4+/CD8+ ratio, IL-2 and INF-gamma production. Moreover, proanthocyanidin strongly enhanced the anti-tumour effect of doxorubicin and the above immune responses, and completely eliminated myocardial oxidative stress induced by doxorubicin. In conclusion, intragastric administration of proanthocyanidin could enhance the anti-tumour activity of doxorubicin and ameliorate doxorubicin-induced myocardial oxidative stress and immunosuppression in tumour-bearing mice.