The hydrolytically unstable polyphosphazenes, poly [(imidazolyl) (methylphenoxy) phosphazenes] and poly [ethyl glycinato) (methylphenoxy) phosphazenes], were studied as potential polymeric supports for cells in tissue regeneration. For bone repair, their specific function would be to support osteoblast growth, forming a bone-polymer matrix. MC3T3-E1 cells (an osteogenic cell line) were seeded onto polymer matrices and cell adhesion and growth as well as polymer degradation were examined. Both imidazolyl- and ethyl glycinato-substituted polyphosphazenes supported the growth of MC3T3-E1 cells. An increase in the content of the imidazolyl side group resulted in a reduction in cell attachment and growth on the polymer surface and an increase in the rate of degradation of the polymer. In contrast, substitution with the ethyl glycinato group favored increased cell adhesion and growth and also an increase in the rate of degradation of the polymers. Thus, the polyphosphazenes represent a system whereby cell growth and degradation can be modulated by varying the nature of the hydrolytically unstable side chain. This in vitro evaluation suggests that the polyphosphazenes may be suitable candidate biomaterials for the construction of a cell-polymer matrix for tissue regeneration.