This study was undertaken to determine the direct effects of extracts derived from Porphyromonas gingivalis on bone formation and mineral resorption in an osteogenic/ osteoclastic cell in vitro co-culture model. Osteogenic bone marrow derived stromal cells were isolated from 18-day old embryonic chickens, while osteoclastic cells were isolated from laying white Leghorn hens on calcium deficient diets. Osteoclastic cells (5 × 105) were seeded onto mineral thin films and suspended above osteogenic cells (1 × 104) already plated on the bottoms of tissue culture plate wells. Sonicated P. gingivalis 2561 extracts were prepared from whole bacterial cells and added in varying proportions (0 to 2 μg/ml) to the co-culture growth medium. These co-cultures, and appropriate mono-culture controls, were incubated for a further 4 days. Parameters of bone forming cell activity including alkaline phosphatase activity, calcium and inorganic phosphate accumulation were performed on the osteogenic cells. Mineral substrate resorption by osteoclastic cells was assessed morphometrically. In their respective mono-cultures, the addition of P. gingivalis sonicate to the culture medium had no effect on osteoclastic mineral resorption, but significantly inhibited osteogenesis (up to 45%; P <0.05). In co-cultures, however, the sonicate induced significant increases in mineral resorption (up to 70%; P<0.05), whereas bone forming cell activity was still inhibited, although to a significantly lesser extent than in mono-cultures (up to 25%; P <0.05). These results suggest that P. gingivalis sonicate induced up-regulation of mineral resorption may be mediated via osteogenic cells. J Periodontol 1998; 69:1263–1270.