Core size of the core and the nature of semiconductor CdSe/ZnS nanoparticles (NP’s) were the subject of the research to understand main reasons of the dynamics of “living cell/nanoparticle” interactions. Core size of NP (with a given type of covering) has a significant impact on their uptake by cells. Nanoparticles with core size 1.7 nm entrapped efficiently when cysteine and mercasptoundecanoic acid are used as coatings. However, the factor of the size of NP’s is not always determinable. Indeed, NP’s coated with cysteamine were captured by cells to a small extent onto the contrary to fluorescent nanoparticles covered with cysteine and mercaptoundecanoic acd. Additional modification of NP shell with antibody allows labeling of target cells that open perspectives for disease diagnostics.