The presence of anti-polyethylene glycol (PEG) antibodies in the systemic circulation might have potential implications for the therapeutic activity of PEGylated products in vivo in the clinic. In order to study the effect of pre-existing anti-PEG antibodies on the in vivo fate and the therapeutic efficiency of PEGylated therapeutics, we developed a BALB/c mouse model by virtue of the intraperitoneal (i.p.) inoculation of hybridoma cells (HIK-M09 and HIK-M11), secreting monoclonal anti-PEG IgM, mimicking the presence of pre-existing anti-PEG antibodies in the blood. In the model, the titers of anti-PEG IgM in the blood increased as a function of hybridoma cells numbers and time after i.p. inoculation. The in vivo levels of anti-PEG IgM decreased in a dose-dependent manner, following i.v. administration of empty PEGylated liposomes. C26 tumor-bearing mice with measurable levels of anti-PEG IgM, receiving i.v. injection of DiR-labeled empty PEGylated liposomes, showed lower levels of liposomal tumor accumulation and higher levels of liver and spleen accumulation, compared to C26 tumor-bearing mice without measurable anti-PEG IgM. This specifies that the presence of anti-PEG IgM in the murine circulation induced accelerated blood clearance of PEGylated liposomes and reduced their tumor accumulation. The biodistribution and antitumor efficacy of commercially available doxorubicin (DXR)-containing PEGylated liposomes, Doxil®, were scrutinized in the anti-PEG IgM mouse model. In C26 tumor-bearing mice having circulating anti-PEG IgM, at 24 h after injection almost no DXR was observed in blood and tumor, and increased DXR accumulation was observed in spleen and liver, compared to tumor-bearing mice with no circulating anti-PEG IgM. The antitumor efficacy of Doxil® was significantly compromised in the C26 tumor-bearing mice in the presence of anti-PEG IgM. These results demonstrate that the anti-PEG IgM mouse model could be a useful prognostic indicator for the therapeutic effectiveness of different formulations of PEGylated therapeutics in pre-clinical studies.