The enzymatic dephosphorylation of the magnetic resonance imaging contrast agent Teslascan was studied in in vitro experiments with acid phosphatase (prostatic, from human semen) and alkaline phosphatase (from human placenta). The active component, MnDPDP (manganese (II)-N,N'-dipyridoxylethylenediamine-N,N'-diacetate-5,5'-bis(phosphate), was dephosphorylated by both enzymes to the monophosphate MnDPMP and the totally dephosphorylated compound MnPLED. The corresponding zinc compound, ZnDPDP (which is a result of in vivo metabolism), was also dephosphorylated by both enzymes to ZnDPMP and ZnPLED. In separate experiments, both enzymes dephosphorylated MnDPMP and ZnDPMP. With the same amount of enzyme units, alkaline phosphatase was almost four times more active than acid phosphatase in dephosphorylating MnDPDP and ZnDPDP with only minor differences whether the substrate contained Mn or Zn. A similar difference in enzymatic activity was seen with the monophosphates, MnDPMP and ZnDPMP. This, taken together with the approximately 50 times higher activity of alkaline phosphatase than acid phosphatase in serum shows that alkaline phosphatase is responsible for most of the dephosphorylation of MnDPDP and its metabolites in vivo.