The purpose of this study was to monitor the expression of matrix metalloproteinase-1 (MMP-1) and tissue inhibitor of metalloproteinase-1 (TIMP-1) produced by an osteoblastic cell line MG63 stimulated with Prevotella nigrescens lipopolysaccharides (LPS), and to compare the level of secretion before and after the P. nigrescens LPS was treated with calcium hydroxide [Ca(OH)2]. The underlying hypothesis is that the balance between MMP and TIMP secretion is the key to an understanding of the host degradative pathways involved in the pathogenesis of bacterial derived pulpal and periapical diseases. Confluent monolayers of MG63 human osteosarcoma cells were exposed to varying concentrations of P. nigrescens or Escherichia coli LPS. Alternately, confluent cultures were exposed to 10 microg/ml of bacterial LPS pretreated with Ca(OH)2 (12.5 mg/ml) for 72 hours. At the end of the experimental period, total RNA was extracted and real-time quantitative polymerase chain reaction (PCR) was performed for MMP-1, TIMP-1, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). The results showed that the expression of MMP-1 mRNA was low and invariant for the experimental period in the negative controls. However, exposure to P. nigrescens LPS increased expression after 48 hours. Expression of TIMP-1 mRNA was highly increased at 24 and 48 hours with lower concentrations of LPS in contrast to a suppression with a concentration of 10 microg/ml. Treatment of P. nigrescens LPS with Ca(OH)2 resulted in a down-regulation of MMP-1, whereas pretreated E. coli LPS demonstrated no stimulatory activity for MMP-1 gene expression. Both types of LPS when pretreated with Ca(OH)2 induced slightly up-regulated expression of TIMP-1.