A simple colorimetric assay using MTT has been developed to monitor mammalian cell survival and proliferation in vitro. In this study we used a clonal fibroblastic cell line (RPC-C2A) from rat incisal dental pulp to examine the effectiveness of the colorimetric assay to test for the toxicity of eugenol, which is frequently used to treat inflammed dental pulp. A technical problem encountered was the insolubility of MTT formazan, produced by the activity of mitochondria dehydrogenases. Dimethyl sulfoxide (DMSO) seemed to be the best solvent. Doses of eugenol causing a 50% inhibition in the colorimetric assay were calculated as 0.6 mM and 1 mM for cells in the growing phase and for cells at confluence, respectively. These values exist in the concentration range reported in the previous studies. Although the correlation between spectrophotometric absorbance and cell number was not completely linear, this method could be used effectively as a simple preliminary assay to test for the toxicity of dental drugs and materials.