The objective of this study was to investigate the presence of a contractile actin isoform, α-smooth muscle actin, in annulus fibrosus cells in situ and in two and three-dimensional cultures. Annulus fibrosus cells were isolated from healthy adult dogs, serial passaged, and then injected into porous collagen-glycosaminoglycan copolymers consisting of either type-I or type-II collagen. α-Smooth muscle actin was detected in the cells in tissue samples and in culture by immunohistochemistry. The number of cells and glycosaminoglycan content of the matrices were determined after 1, 7, and 14 days, and the diameters of the specimens were measured every 2 days. Although few annulus fibrosus cells in vivo displayed the presence of the α-smooth muscle actin isoform, most cells in two-dimensional culture demonstrated this phenotype. The contractile behavior of these cells was shown by the cell-mediated contraction of type-I collagen- glycosaminoglycan scaffolds after 8 days in culture. Glycosaminoglycan production was not significantly different in the seeded type-I matrices than in the unseeded matrices, whereas the seeded type-II matrices had a significant increase in glycosaminoglycan production between days 1 and 14 compared with the unseeded controls. This is the first report of both the expression of the contractile α-smooth muscle actin isoform in intervertebral disc cells and the ability of the cells to contract a collagen matrix. This finding could aid in better understanding the nature of cells in the annulus.