It is an established fact that T-cell responses of fetal origin to inhalant allergens are present in most cord blood samples. These immune responses could be explained by trans-placental passage of peptides, either as free antigens or in complexes with immunoglobulin G (IgG), providing the fetus with a trigger for priming the T-cell system already present in utero. The aim of this study was to investigate the presence of the major cat allergen, Fel d 1, in complexes with IgG in cord blood and maternal sera. Serum samples from 75 mothers (38 allergic, 37 non-allergic), and cord blood from their infants, were investigated for the presence of Fel d 1–IgG immune complexes (ICs) by using an amplified enzyme-linked immunosorbent assay (ELISA). Three monoclonal antibodies to Fel d 1 were used for coating. The specificity of the method was confirmed by inhibition experiments. ICs of Fel d 1–IgG were detected in the sera of 45% allergic and 49% non-allergic mothers, and in, respectively, 34% and 41% of their infants. Therefore, neither the prevalence nor the level of ICs were affected by maternal allergy. Low levels of trans-placentally transferred ICs can provide the fetus with a signal for the priming of T-cell responses to inhalant allergens. However, this is not necessarily related to allergic disease.