For nanomedical targeting and drug delivery purposes, the noncovalent assembly of polymer building blocks into defined nanostructures is an intense area of research. One of the key assets desirable to know for the potential nanocarrier is the stability under conditions close to those in application scenarios. Here, a series of polymer building blocks based on poly(ethylene glycol) (PEG), which comprise a functional end group facilitating self-assembly into supramolecular polymer bottlebrushes (SPBs), is hydrodynamically studied. The building blocks, and consequently the assemblies, are labeled with a cyanine5 (Cy5) dye enabling selective tracing of the materials in human serum (HS) in analytical ultracentrifugation (AUC) experiments. Our experiments reveal a long-term stability of the noncovalent assemblies over one month of storage of the materials in HS at body temperature. At the same time, the interaction of some of the Cy5 moieties with the transport protein human serum albumin (HSA) is evidenced.