In this study, gas chromatography/mass spectrometry revealed the presence of stoichiometric amounts of myo-inositol in association with serum corticosteroid-induced isozyme of alkaline phosphatase (CALP) in canine serum. Such remnants are consistent with prior membrane attachment of serum CALP and its release into serum by endogenous phospholipase activity. Serum CALP was further shown to behave similarly to CALP released from hepatocyte membranes by glycosyl phosphatidylinositol phospholipase D (GPI-PLD) and differently from CALP solubilized by GPI-phospholipase C (PLC) on both native polyacrylamide gel electrophoresis and Western blot analysis using anti-cross-reacting determinant antibody. In addition to bile canalicular surfaces, CALP activity was found over hepatocyte sinusoidal surfaces by histochemical staining of canine liver sections. A significantly higher ratio of CALP to total alkaline phosphatase activity was observed in serum as opposed to bile in 10 of 11 paired serum and bile samples from dogs. This suggested that bile is not likely to be the source of serum CALP and is consistent with the release of CALP from hepatocyte basolateral surfaces directly into serum. It was concluded that serum CALP was once membrane bound and was released by phospholipase activity into serum. Our findings are consistent with release of CALP from the sinusoidal surfaces of hepatocytes into serum either by endogenous GPI-PLD activity or release by GPI-PLC followed by modification of the phosphatidylinositol remnant in vivo.