In the present study, we evaluated the toxic effects of mercaptoacetic acid (MM)-capped CdSe QDs and CdSe/ZnS QDs to particle-ingesting model ciliated protozoa Tetrahymena thermophila BF5 (T. thermophila BF5) by using a TAM air isothermal microcalorimeter. These results suggested that both MAA-CdSe QDs and MM-CdSe/ZnS QDs were indeed acutely toxic for T. thermophila BF5 growth in a dose-dependent manner, and the toxicities of both MAA-CdSe QDs and MAA-CdSe/ZnS QDs increased dramatically after UV irradiation due to the liberation of more toxic Cd2+, which indicated that the toxicity of MAA-CdSe/ZnS QDs was less than that of MAA-CdSe QDs. Furthermore, the toxicity of different ligands-capped CdSe/ZnS QDs on T. thermophila BF5 was also investigated. The uptake of MAA-CdSe/ZnS QDs and adenosine 5'-monophosphate (AMP)-CdSe/ZnS QDs by cells and the morphological change during the process of T. thermophila BF5 growth incubated with these QDs were further studied by fluorescence inverted microscopy. (C) 2012 Elsevier B.V. All rights reserved.