OBJECTIVE The aim of this study was to investigate the effect of titanium tetrafluoride solution on L929 fibroblasts by scanning electron microscopy. Titanium tetrafluoride was then compared with sodium fluoride and acidulated phosphate fluoride. STUDY DESIGN Cells were treated with fluoride solutions for 1 minute either directly, through a filter membrane with a pore size of 0.4-micron, or indirectly, through dentin disks; they were then investigated at an electron microscopic level. RESULTS Fluoride application on smeared dentin disks showed fewer cytotoxic effects on fibroblasts than application on nonsmeared dentin disks. Acidulated phosphate fluoride and titanium tetrafluoride appeared to be more cytotoxic than sodium fluoride. Because all fluoride solutions used in this study contained the same fluoride concentration, pH was considered to be the main factor causing the higher toxicity. CONCLUSION Because these solutions demonstrated toxicity in vitro, they must be further evaluated under in vivo conditions to ascertain their clinical safety.