Alkaline phosphatase (ALP), which was discovered in 1907, is extensively reviewed. The first section deals with biochemical aspects of ALP, e.g. the anchorage of ALP to cell membranes via a phosphatidylinositol linkage, the charge and molecular mass heterogeneities and their causes, and methods for the separation and analysis of ALP isoforms using the newest electrophoresis techniques, such as affinity electrophoresis with wheatgerm lectin and isoelectric focusing in immobilized pH gradients. The second section deals exclusively with the clinical implications of ALP analysis in diseased states, e.g. cholestasis, hyper- and hypophosphatasemias and neutrophil ALP. Extensively discussed is the involvement of ALP in numerous bone diseases. The role of ALP and its isoforms in tumours and their applicability as potential tumour markers is critically evaluated.