The role of macrophages in nanomedicine is widely recognized, but systemically administered drug nanocarriers are rapidly absorbed and eliminated by the mononuclear phagocyte system, consisting of resident macrophages, primarily in the liver. As a result, most encapsulated drugs are eliminated from circulation, resulting in unwanted side effects. Peritoneal macrophages, on the contrary, by ingesting nanoparticles and migrating to tumor cells, can promote the antitumor effect. This article describes the preparation of complexes based on upconversion nanoparticles (UCNP) coated with nitrosonium tetrafluoroborate (NOBF4) with a protein corona (PC) from native and denatured bovine serum albumin (BSA and dBSA). The efficiency of UCNP-protein complexes uptake by mouse peritoneal macrophages has been demonstrated. The use of this approach is a promising area of oncology, since instead of inefficient systemic intravenous delivery, peritoneal delivery is used, which can become the key to solving the problem of peritoneal cavity cancers.