Cells that are taken from the nucleus pulposus (NP) and that are allowed to proliferate in monolayer cultures often exhibit changes in their cell morphology and matrix-protein synthesis. However, whether concomitant alterations occur with respect to their mRNA levels for collagen I (CI), collagen II (CII) and aggrecan (AGG) is unclear. In this study, human NP cells from seven individuals were cultured in monolayers and specific mRNAs for CI, CII and AGG were quantified by real-time polymerase chain reaction in fresh NP tissue and during four passages of NP-cell culture. In addition, the presence of CI, CII and AGG protein was determined by immunofluorescence staining of NP cells. We found a significant reduction of CI, CII and AGG mRNA after the initiation of culture in DMEM compared with mRNA levels in fresh NP tissue. During passages 2--4, no further reduction of mRNA levels for CII and AGG was observed. The mRNA level for CI was reduced significantly with duration of culture. Immunofluorescence staining of cultured NP cells revealed expression of CI, CII and AGG protein during the whole culture period. Our data thus demonstrate a reduction of specific mRNA for matrix proteins during the initiation of NP-cell culture but the stable expression of the key matrix proteins, CII and AGG, during further expansion of the cells in monolayers, suggesting no functional changes occur in cultured NP cells.