Cisplatin as a chief chemotherapy has nephro-toxicity and so we have tried to develop a novel antitumor drug based on a combination of cobalt metal ion with an organic moiety. The antitumor activity of the complex was tested in vitro and in vivo against murine Ehrlich ascites carcinoma (EAC). Antioxidant capacity and nucleic acids content were determined. Cobalt(ii) diacetyl monoxime-2-hydrazinopyridine complex significantly diminished tumor load. It decreased the tumor proliferation rate and obviously increased the life span of EAC-bearing mice. It reversed the haematological parameters back towards normal, reduced liver enzymes and urea, and increased albumin and total protein. Antioxidant parameters levels were reversed towards normal. An assessment was conducted by comparing these results with those obtained using the standard drug, cisplatin. The results suggest that the cobalt complex can be considered as a potent anticancer agent as it showed appreciable antitumor activity in EAC-bearing mice that was almost analogous to that of the reference standard, cisplatin.