The present study was performed to investigate the effects of endotoxins from periodontopathic bacteria on collagen metabolism. Endotoxins were extracted from Bacteroides gingivalis 381 and Bacteroides intermedius ATCC 25611 using the hot-phenol method. A commercially available endotoxin from Escherichia coli 0111: B4 was also used as a control. Human gingival fibroblasts (Gin-1, ATCC CRL 1292) were maintained with DMEM containing 10% FBS. When the fibroblasts became confluent they were exposed to each of the endotoxins in various concentrations (0, 5, 10, 15, 20 micrograms/ml). The effects of these endotoxins on the collagen metabolism of the fibroblasts were assessed on the basis of collagen synthesis and collagenase activity. The former was assessed by 3H-proline incorporation and bacterial collagenase digestable protein. The latter was assessed by fibril assay. The results were as follows: There was no change in glucose consumption, cell viability or morphology under the light microscope when the concentration of endotoxin was 20 micrograms/ml or less. 3H-proline incorporation into protein and collagen synthesis were inclined to decrease. Endotoxin from B. gingivalis resulted in an acceleration of collagenase activity. There findings indicate that the endotoxins from these bacteria might affect collagen metabolism early in gingivitis in vivo.