Alkaline phosphatase (ALP) catalyzes the hydrolysis of phosphate esters and it seems to be a prerequisite for normal skeletal mineralization. Also, ALP is the most widely recognized marker of osteoblast phenotypes. By a tissue regenerative technique called Guided Bone Regeneration (GBR), it is possible nowadays to regenerate small bony defects. The aim of the present study was to investigate early events in bone healing and neogenesis by studying histochemically the temporal and local appearance of the marker Alkaline Phosphatase (ALP) in a GBR model system. Nine healthy volunteers (5 males, 4 females, mean age 31.7 years) participated in the experiment. After raising a mucoperiosteal flap from the mandibular second molar to the retromolar area in each volunteer, a hollow titanium test cylinder was placed into a congruent bony bed and the coronal end of the cylinder was closed with an ePTFE-membrane. Then the flap was adapted and sutured to obtain primary wound closure. After 2, 7 and 12 weeks, the regenerated tissue within the cylinders was harvested. Histologically, ALP activity was observed associated with the osteoid seams in the very basal part of the regenerate where new bone trabeculae were in the process of being formed. More coronally, large round cells seemed to secrete an ALP-positive substance since in the center of such cell clusters strong ALP activity located extracellularly was detected. In the present experiment, ALP seemed to have been an early sign of osteoblast secretion of a matrix which subsequently was determined to become osteoid. ALP activity was never seen isolated within connective tissue and away from bone. This is an indication that its source is linked to existing bone. The present study has documented for the first time the appearance of ALP activity in guided bone regenerations in humans. It has revealed that: 1) Osteogenesis in guided bone regeneration is preceded by localized, marked expression of ALP in an organized connective tissue environment. 2) Bone neogenesis is an early event in this experimental setup and may be detected already 2 weeks after wounding. 3) Expression of ALP and subsequent bone neogenesis is originating from and topographically linked to pre-existing bone structures.