Pressed juices of the aerial parts of Echinacea purpurea are used as non-specific immunostimulants, and arabinogalactan-proteins (AGPs) have been shown to be part of the active principle. Monoclonal antibodies against an AGP from pressed juice of Echinacea purpurea with complement-stimulating activity have been established by means of hybridoma techniques. To test the specificity of the antibodies, several other arabinogalactan-proteins from suspension cultures of Echinacea purpurea, the roots of Echinacea pallida, the aerial parts of Rudbeckia hirta, the roots of Baptisia tinctoria and gum arabic as well as an arabinogalactan from larch wood were tested in a competitive ELISA for cross reactivities. Chemical modifications at the periphery of the AGP molecules either by reduction of uronic acids or by dearabinosylation had no influence on the reactivity of the molecules towards the antibodies. For further characterization of the epitope, different Ara-Gal-oligosaccharides were used as antigens. A hexasaccharide consisting of a backbone of four molecules of 6-linked beta- D-Gal p, the second and the fourth of them branched at O-2 to an alpha- L-Ara f residue showed weak but reproducible cross reactivity, indicating that the antibodies may be at least in part directed to the carbohydrate moiety of the AGP. Testing of anti-AGP antibodies JIM 8 and LM 2 revealed good reactivity of LM 2 with the Echinacea AGP, whereas Jim 8 showed only very weak interaction.