Specificities of IgE and IgG4 antibodies in 12 cat-allergic patients were compared with respect to their reactivity towards 3 IgE-binding synthetic peptides of Felis domesticus allergen 1 (Fel dI): peptides 25-38 and 46-59 of chain 1 and peptide 15-28 of chain 2. Peptides were coupled to Sepharose and anti-Fel dI antibodies were isolated by affinity chromatography. Fel dI-specific IgE- and IgG4 antibody activity in the peptide eluates was measured using Fel dI binding assays. Fel dI-specific IgE/IgG4 ratios in the eluates from peptide-Sepharose were determined and compared with the IgE/IgG4 ratios in the eluates from Fel dI-Sepharose. The mean ratio Fel dI-specific IgE/IgG4 in the peptide eluates (0.84; range 0.06-4.6) was significantly higher than the mean ratio in the eluates from Fel dI-Sepharose (0.31; range 0.13-1.1), demonstrating a higher reactivity of IgE antibodies with the peptides, compared to IgG4. These results indicate differences between the B cells producing IgE antibodies and the B cells producing IgG4 antibodies.