To understand the interactions among components of the immune/inflammation response, we studied the effects of immunoglobulins on the phosphatase activity of alkaline phosphatase in vitro. Bovine intestinal alkaline phosphatase was incubated with substrate in the presence of allotypic and xenotypic immunoglobulin. We found that bovine but not rabbit immunoglobulin enhanced the phosphatase activity of bovine intestinal alkaline phosphatase. Similarly, human but not bovine immunoglobulin G enhanced human placental alkaline phosphatase activity. By enhancing alkaline phosphatase activity, immunoglobulins bound to alkaline phosphatase may assist physiologic transport functions and enhance resolution of the inflammatory response. Further, in clinical conditions with high immunoglobulin concentrations, the serum alkaline phosphatase recorded may have spuriously high values.