The effect which a visible light-curing periodontal dressing material based upon a urethane dimethacrylate resin has upon HeLa cells and fibroblasts in its uncured and fully cured states has been studied in vitro. Fully-cured material has no effect on either cell type. Uncured material produces a surrounding zone of growth inhibition and cell death on direct contact. Inhibition is caused by the release into the medium of substances toxic to cells. However, this growth inhibition is overcome in time (5 days). It is suggested that partly-cured material containing residual free monomer in contact with a healing gingival site could impede rapid repair. Nuclear magnetic resonance microimaging has shown that in places where there is a greater depth of material (e.g., interproximal regions) this underlying material may be partly cured. Increasing the exposure to the curing light will prevent (or minimize) the presence of partly-cured material; the fully-cured material being compatible with the cells.