The host specificity of Tricholoma species in ectomycorrhiza formation was exploited to show specific accumulation of hydrophobin protein using polyclonal antiserum raised against the hydrophobin Sc3 from the homobasidiomycete Schizophyllum commune. This small, hydrophobic protein is found in cell walls of monokaryotic aerial mycelium of S. commune. In immunofluoresence studies of irt vitro synthesized ectomycorrhiza of Tricholoma terreum with pine and fir a specific accumulation in the Hartig' net could be shown in the compatible interaction of T. terreum with Pinus sylvestris, while the incompatible interaction of T. terreum with Picea abies gave no evidence of hydrophobin accumulation. The hyphal mantle was stained in both cases. A role for the Tricholoma hydrophobin in Hartig' net assembly and cell/cell contact of both hyphal/hyphal contact and contact of hyphae to the lignin containing plant cell wall is discussed.