For the systemic application of nucleic acids such as plasmid DNA and small interfering RNA, safe and efficient carriers that overcome the poor pharmacokinetic properties of nucleic acids are required. A cationic liposome that can formulate lipoplexes with nucleic acids has significant promise as an efficient delivery system in gene therapy. To achieve in vivo stability and long circulation, most lipoplexes are modified with PEG (PEGylation). However, we reported that PEGylated liposomes lose their long-circulating properties when they are injected repeatedly at certain intervals in the same animal. This unexpected and undesirable phenomenon is referred to as the accelerated blood clearance (ABC) phenomenon. Anti-PEG IgM produced in response to the first dose of PEGylated liposomes has proven to be a major cause of the ABC phenomenon. Therefore, in a repeated dosing schedule, the detection of anti-PEG IgM in an animal treated with PEGylated lipoplex could be essential to predict the occurrence of the ABC phenomenon. This chapter introduces a method for the evaluation of serum anti-PEG IgM by a simple ELISA procedure, and describes some precautions associated with this method.