Aiming at the use of vitamin B12 as a drug delivery carrier for cytotoxic agents, we have reacted vitamin B12 with trans-[PtCl(NH3)2(H2O)]+, [PtCl3(NH3)](-) and [PtCl4](2-). These Pt(II) precursors coordinated directly to the Co(III)-bound cyanide, giving the conjugates [(Co)-CN-(trans-PtCl(NH3)2)]+ (5), [(Co)-CN-(trans-PtCl2(NH3))] (6), [(Co)-CN-(cis-PtCl2(NH3))] (7) and [(Co)-CN-(PtCl3)](-) (8) in good yields. Spectroscopic analyses for all compounds and X-ray structure elucidation for 5 and 7 confirmed their authenticity and the presence of the central "Co-CN-Pt" motif. Applicability of these heterodinuclear conjugates depends primarily on serum stability. Whereas 6 and 8 transmetallated rapidly to bovine serum albumin proteins, compounds 5 and 7 were reasonably stable. Around 20% of cyanocobalamin could be detected after 48 h, while the remaining 80% was still the respective vitamin B12 conjugates. Release of the platinum complexes from vitamin B12 is driven by intracellular reduction of Co(III) to Co(II) to Co(I) and subsequent adenosylation by the adenosyltransferase CobA. Despite bearing a rather large metal complex on the beta-axial position, the cobamides in 5 and 7 are recognized by the corrinoid adenosyltransferase enzyme that catalyzes the formation of the organometallic C-Co bond present in adenosylcobalamin after release of the Pt(II) complexes. Thus, vitamin B12 can potentially be used for delivering metal-containing compounds into cells.