Titanium dioxide (TiO₂) nanoparticles have a high surface-to-mass ratio and rapidly aggregate in water causing great difficulties for toxicity test exposed to aquatic organisms or other cell lines. This study uses a cell viability kit for routine toxicity test of TiO₂ as well as other nanoparticles which accumulate in the aquatic environment. Tetrahymena immediately endocytoses TiO₂ nanoparticles and stores them in food vacuoles until the particles undergo exocytosis as larger aggregates. However, during the process of endocytosis and exocytosis, TiO₂ particles interfere with cell growth and consequently induce acute toxicity. It exerted high cell growth inhibition at 20 h incubation and induces significant cytotoxic effects. Surprisingly, the effect of TiO₂ decreases at 40 h incubation, due to the recovery of cell growth and reduction of the cytotoxicity of the particles.