It is attractive to use vitamin B₁₂ as a carrier for targeted delivery of cytotoxic agents such as platinum complexes owing to the high demand for vitamin B₁₂ by fast proliferating cells. The basic {B₁₂-CN-Pt(II)} conjugates are recognized by intracellular enzymes and converted to coenzyme B₁₂ in an enzymatic adenosylation assay. The reductive adenosylation of {B₁₂-CN-Pt(II)} conjugates leads to the release of the Pt(II) complexes; thus, {B₁₂-CN-Pt(II)} conjugates can be considered as prodrugs. It is important not only to elucidate the activity of the cisplatin-B₁₂ conjugates, but also to understand the mode of action on a molecular level. Chemical reduction of {B₁₂-CN-Pt(II)} conjugates with cobaltocene yielded cob(II)alamin and induced release of the corresponding Pt(II) species. Kurnakov tests and coordination of 2'-deoxyguanosine or GMP to the released Pt(II) complexes allowed isolation and characterization of Pt(II) complexes as released during enzymatic adenosylation. The biological activity of these Pt(II) complexes was evaluated. Since the cleaved Pt(II) complexes show cytotoxicity, the {B₁₂-CN-Pt(II)} conjugates can be used for specific targeting of cancer cells and therapeutic drug delivery. Preliminary in vitro cytotoxicity studies indicated lower activity (IC(50) between 8 and 88 μM) than found for pure cisplatin. Since active transport and receptor-mediated uptake limits the intracellular {B₁₂-CN-Pt(II)} concentration, comparison with pure cisplatin is of limited use. We could show that the Pt(II) complexes cleaved from B₁₂ exerted a cytotoxicity comparable to that of cisplatin itself. Cytotoxicity studies in vitamin B₁₂ free media showed a dependence on the addition of transcobalamin II for B₁₂-Pt(II) conjugates.