In the formulation of peptide- and protein-based drugs, it is important that the pharmaceutical excipients used do not potentiate possible immunogenic properties of the drug substance. Polyethylene glycols (PEGs) are widely used excipients e.g. in parenteralia and in formulations for nasal application. The immunomodulating properties of PEG 400 were investigated in this study using hen egg ovalbumin (OA) as the model immunogen. OA was dissolved in saline, 10% PEG 400 in saline or undiluted PEG 400 and injected subcutaneously into the neck region of BALB/cJ mice. The levels of OA-specific IgE, IgG1 and IgG2a antibodies were measured. The 10% solution of PEG 400 did not have any immunomodulating properties, whereas the undiluted product gave rise to immunosuppression when compared with the saline control. Neither 10% nor the 100% PEG 400 preparation possessed adjuvant activity under the conditions of the study.