New endodontic materials with polymer bases may be more difficult to evaluate in cell cultures in vitro than conventional zinc oxide-eugenol cements. In order to study the morphological changes taking place in cells exposed to such materials, L929 cells and human periodontal fibroblasts were observed using scanning electron microscopic and transmission electron microscopic techniques. The morphological changes of the cells were correlated to the quantitative results observed simultaneously in cytotoxicity studies using the radiochromium release method.