The Authors demonstrate that serum alkaline phosphatase (s-ALP) activity in vitro is inhibited by NaDDTC. This may be related to the NaDDTC chelating action on zinc enzyme. The trend of the phenomenon follows an exponential pattern. The activity of the isozyme pool was restored after removal from medium of NaDDTC by dialysis. The NaDDTC concentration able to inhibit in vitro enzymatic activity was nearly one thousand times higher than that found in vivo, in experimental animals (rabbit). The Authors conclude that the in vivo activity of NaDDTC in different enzymatic systems, such as ALP and other metallo-dependent enzymes, cannot be explained by its chelating action alone, but also by its influence on other systems. At present investigations in this field are in progress in our laboratory.