Local irritation of gingival tissue by plaque is among the factors which affect development of gingival overgrowth in patients undergoing chronic phenytoin (PHT) therapy. Variability in the cytotoxicity of plaque components or of plaque substances plus PHT and/or its metabolites toward gingival fibroblasts may relate to whether gingival overgrowth forms in a particular patient. Fibroblasts from healthy and overgrown gingivae were incubated with (a) PHT and its major human metabolite, 5-(p-hydroxyphenyl)-5-phenylhydantoin (HPPH), (b) microbial and “amplified” plaque extracts, and (c) microbial and “amplified” plaque extracts plus PHT and HPPH. Cell numbers and cell-associated protein were determined for each incubation preparation. A wide range in cytotoxic response to a particular microbial or plaque extract occurred among cell strains. Plaque extracts from different subjects had variable cytotoxicity toward a cell strain. The differences among fibroblast strains in response to an extract and the variability in cytotoxicity of different plaque extracts toward a cell strain were not related to their source from normal or overgrown gingivae. Cell numbers and cell-associated protein were similar for incubation mixtures containing extracts with and without PHT and HPPH. These data do not show differences among cytotoxicity levels of plaque extracts, the response of particular gingival fibroblast strains to plaque components, or interaction between drugs and certain plaque samples which explain development of gingival overgrowth in some subjects receiving chronic PHT therapy.