A single intravenous administration of polyethylene glycol-coated (PEGylated) bovine serum albumin (BSA) and ovalbumin (OVA) elicited an anti-PEG immunoglobulin M (IgM) response, similar to that from PEGylated liposomes, although the administration did not elicit specific neutralizing antibodies to BSA and OVA. A cross-reactivity was observed between anti-PEG IgMs elicited by PEG-BSA and PEGylated liposomes. The anti-PEG IgM level induced by PEGylated proteins (BSA and OVA) reached the maximum at day 5 following intravenous injection. This production pattern was consistent with that induced by PEGylated liposomes. Splenectomy suppressed the anti-PEG IgM response against PEG-BSA and PEGylated liposomes. These observations relating PEG-BSA and PEGylated liposomes indicate that PEGylated proteins might promote the immune responses against PEG with a mechanism similar to that of PEGylated liposomes. In addition, a single intravenous administration of PEGylated adenovirus (PEG-Ad) also elicited an anti-PEG IgM response in a PEG-modification ratio dependent manner. To the best of our knowledge, this is the first report showing that an intravenous administration can elicit an anti-PEG IgM response against PEGylated substances. It appears that anti-PEG IgMs can be produced by the systemic administration of a PEGylated substance and may limit the efficacy of PEGylated substances such as proteins, Ad vector and nanoparticles, due to a cross-reactivity seen in some patients. The immunogenicity of PEGylated substances is usually tested against those very substances, rather than against covalently attached PEG. Our study suggests that the PEG immunogenicity of PEGylated therapeutic agents and particles merits further investigation.