The purpose of the present report was to evaluate on a clinical and histological level the effect of hydroxylapatite (HA)--Interpore 200--implanted into various types of periodontal defects. Additionally in-vitro studies with enzymatically released calvarial cells from fetal rats were performed in the organoid culture system. The clinical data 12 months after surgery revealed a greater reduction of probing depths (PD) and gain of attachment level in the implanted sites (59 sites) when compared to the sites treated by open flap curettage (40 sites). When the results were analyzed according to the initial PD the measurements showed no significant difference between the two modalities of treatment in moderately deep pockets (PD less than or equal to 6 mm) in contrast to deep and extremely deep pockets (PD = 7-9 mm and PD greater than 9 mm). In order to collect data about the type of periodontal wound healing biopsies, respectively block sections were taken 6-12 months after therapy in 6 patients and evaluated by light microscopy. Three forms of tissue reaction in relation to the implants were observed. The first consisted of a fibrous connective tissue encapsulation of the implants, the second showed organized bone tissue around the HA, while the third type of tissue reaction consisted of a deposit of mineralized tissue on the surface limited in width, which was interpreted as cementum formation. Analysis of the block sections revealed new connective tissue attachment to a certain extent and bone/cementum-like tissue formation. The possibility of cementum-like tissue formation on HA was confirmed transmission electronmicroscopically by the in-vitro experiments.(ABSTRACT TRUNCATED AT 250 WORDS)