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An investigation has been made of the inhibitory activity of the so-called Hamburger pentapeptide, Asp-Ser-Asp-Pro-Arg, in both the human Prausnitz-Küstner and baboon passive cutaneous anaphylaxis systems. With a dose of 6 μg peptide, administered intradermally 24 h prior to diluted allergic serum from a horse-dandruff-sensitive individual, partial inhibition of wealing was observed in three different normal recipients following the pricking in of specific allergen. In one of the recipients, inhibition of flaring was observed at test sites subsequently challenged with timothy grass pollen allergen. A similar dose of pentapeptide, again injected intradermally 24 h prior to passive sensitisation, was shown to inhibit extravasation in a normal baboon on subsequent challenge with horse dandruff (but not grass pollen) allergen by subcutaneous injection into the upper arm. In vivo tests performed in the forearms of grass-pollen-sensitive individuals failed to provide convincing evidence of displacement of mast-cell-bound IgE antibody by the pentapeptide.