Dendritic polymer conjugates which are useful drug delivery systems for carrying platinum containing anti-tumor agents to malignant tumors are prepared by obtaining a dendritic polymer having functional groups which are accessible to a platinum containing compound capable of interacting with the functional groups, contacting the dendritic polymer with the platinum containing compound, and reacting the dendritic polymer with the platinum containing compound. The dendritic polymer platinates may be administered intravenously, orally, parentally, subcutaneously or topically to an animal having a malignant tumor in an amount which is effective to inhibit growth of the malignant tumor. The dendritic polymer platinates exhibit high drug efficiency, high drug carrying capacity, good water solubility, good stability on storage, reduced toxicity, and improved anti-tumor activity in vivo.