Cancer is one of the leading causes of death. Radiation therapy is an important modality used in cancer treatment being highly cost-effective. Major flaw of radiotherapy is lack of selectivity between cancerous and healthy tissues. Amelioration of radiotherapy by using high-Z nanoparticles as radiation enhancers is one of potential solutions. Gold nanoparticles (AuNPs) are commonly used as radioenhancers. Understanding the interaction between cancer cells and AuNPs is essential in order to achieve best possible radioenhancing effects, while sparing healthy tissues. This work aims to elucidate interactions of ultrasmall (core size: 2.4 nm and hydrodynamic diameter (Dh): 4.5 nm) fluorescently labeled AuNPs with various human cell lines. In this perspective we measured uptake dynamics, characterized route of internalization and time of intracellular retention in various cancer cell lines and fibroblasts. Our results show that uptake dynamics and internalization pathways are strongly cell line-dependant. We also demonstrate that higher proportion of internalized nanoparticles resides in cancer cells, compared to fibroblasts, in in vitro conditions. This work highlights great complexity of cancerous cells and underlines the necessity for excellent knowledge of biological behaviour for each type of cancer. It also emphasizes the major effort needed for efficient cancer treatments and makes an appeal for further development of highly selective nanoparticles in order to hasten their utilization in clinical conditions.