Background: Alkaline phosphatase is a glycoprotein that catalyzes two kinds of chemical reactions: hydrolysis of phosphorus ester breaking P-O bonds and phospho-transfer reactions in which phosphoric group is transferred to an acceptor molecule. In the human body, ALP exists in multiple molecular forms whose heterogeneity is partly due to genetic factors and partly to posttranslational modifications. The aim was to evaluate a total ALP activity and its isoforms in cancer patients with bone and liver metastasis in comparison to healthy controls. Methods: Human serum was collected from 20 healthy individuals, and 20 cancer patients with bone and liver metastases, with metastases confirmed by ultrasound, computerized tomography and a radiology scan. Determination of ALP was done by the endpoint spectrophotometric method. Isoenzymes were determined by heat inactivation method. Results: In cancer patients, the total ALP activity was significantly higher (p< 0.05) compared to healthy controls. In the sera of cancer patients with liver metastases, the remaining ALP activity was two-fold higher in comparison to bone metastases. Conclusion: Determination of ALP isoenzymes is important but a correct clinical interpretation in the context of other analyses is vital for a proper diagnosis of a disease. [Projekat Ministarstva nauke Republike Srbije, br.175056