In a certain stage of development, the performance of nanoparticle- or polymer drug conjugates is tested "in vivo", that is, in mice or rats. Besides pharmaceutical and chemical characterization, the structural characterization of such drug carrier systems in terms of size, size distribution, and shape is typically performed in physiological salt solution prior to animal tests. The present work introduces a simple method based on dynamic light scattering to monitor the particle size in blood serum. Utilizing a model system of pegylated poly-L-lysines (PLL-g-PEOx) of various degrees of pegylation,x, it is demonstrated that large aggregates may form in human serum solution that are not observed in isotonic salt solution. Aggregates of a few hundred nanometers in size were found in mixtures of scrum solution and PLL-g-PEOx with degrees of pegylation <10%, whereas no aggregates are being observed if the degree of pegylation exceeds 20%. The described method may have the potential to become an easy and routine test for drug carrier systems prior to animal applications.