Ultrasmall silver nanoclusters (AgNCs) are a novel type of fluorescent nanoprobes that have aroused a great deal of interest in recent years. In view of many promising applications in biological research, it is of great importance to explore their behavior in the complex biological environment. In this study, interactions of AgNCs with a model protein, human serum albumin (HSA), have been systematically investigated by using a variety of techniques including absorption spectroscopy, steady-state and time-resolved fluorescence, as well as circular dichroism spectroscopy. The results show that the physicochemical properties of both proteins and AgNCs undergo changes upon their interactions; however, it appears that the overall conformation of HSA remains essentially unaffected in the complex. Binding of HSA to AgNCs was assessed by measuring tryptophan fluorescence quenching of HSA by AgNCs. Furthermore, biological implications of protein adsorption were quantitatively explored by evaluating responses of HeLa cells to AgNC exposure through live-cell fluorescence microscopy and a cytotoxicity test, revealing that protein adsorption has a significant effect on the biological response to AgNC exposure.