We have reported that PEGylated liposomes lose their long-circulating properties when injected twice into the same animal within a certain interval (the accelerated blood clearance (ABC) phenomenon). We assumed that this phenomenon was triggered via the abundant secretion of anti-polyethylene glycol (PEG) immunoglobulin M (IgM) in response to the first dose of PEGylated liposomes and that the spleen played an important role in the production of anti-PEG IgM. However, no direct evidence has yet confirmed this suspicion. In the current study, we verified, both in vitro and ex vivo, that spleen cells are indeed responsible for the production of anti-PEG IgM in response to PEGylated liposomes. In this study, spleen cells obtained from either naïve mice or mice pre-treated with PEGylated liposomes induced the production of anti-PEG IgM in a dose- and time-dependent manner, upon incubation with PEGylated liposomes. In addition, we confirmed that among the different fractions of splenic B cells, IgM-positive B cells, rather than CD45R-positive or CD19-positive splenic B cells, which are presumed to be the marginal zone B (MZB) cells, are the major cells producing anti-PEG IgM in the response to stimulation by PEGylated liposomes. These results may provide new insights into the mechanisms underlying the anti-PEG IgM production in response to the stimulation by PEGylated liposomes.