PEGylation, the attachment of polyethylene glycol (PEG) to nanocarriers and proteins, is a widely accepted approach to improving the in vivo efficacy of the non-PEGylated products. However, both PEGylated liposomes and PEGylated proteins reportedly trigger the production of specific antibodies, mainly IgM, against the PEG moiety, which possibly leads to a reduction in safety and therapeutic efficacy of the PEGylated products. In the present study, two monoclonal anti-PEG IgMs — HIK-M09 via immunization with an intravenous injection of PEGylated liposomes (SLs) and HIK-M11 via immunization with a subcutaneous administration of PEGylated ovalbumin (PEG-OVA) were successfully generated. The generated IgMs showed efficient reactivity to mPEG2000 conjugated to 1,2-distearoyl-sn-glycero-3-phospho-ethanolamine (DSPE), PEGylated liposome (SL) and PEG-OVA. It appears that HIK-M09 recognizes ethoxy (OCH2CH2) repeat units along with a terminal motif of PEG, while HIK-M11 recognizes only ethoxy repeat units of PEG. Such unique properties allow HIK-M09 to bind with dense PEG. In addition, their impact on the in vivo clearance of the PEGylated products was investigated. It was found that the generated ant-PEG IgMs induced a clearance of SL as they were intravenously administered with SL. Interestingly, the HIK-M11, generated by PEG-OVA, induced the clearance of both SL and PEG-OVA, while the HIK-M09, generated by SL, induced the clearance of SL only. We here revealed that the presence of serum anti-PEG IgM and the subsequent binding of anti-PEG IgM to the PEGylated products are not necessarily related to the enhanced clearance of the products. It appears that subsequent complement activation following anti-PEG IgM binding is the most important step in dictating the in vivo fate of PEGylated products. This study may have implications for the design, development and clinical application of PEGylated products and therapeutics.