Several isolates of dermatophytes covering 13 different species were studied for their haemagglutination activity with erythrocytes of different origins (human, bovine, sheep, mouse and rabbit). The best results were obtained with rabbit erythrocytes. The specific haemagglutination activity of the fungal soluble extracts seemed to be closely related to the ecology of the fungi. Among the different carbohydrate (ose, polyol, osamine) and glycoprotein solutions used in the inhibition experiments, only N-acetyl-neuraminic acid and a sialoglycoprotein, the bovine submaxillary mucin (mucin type 1), showed an inhibiting activity, demonstrating the lectin nature of the haemagglutinins and the importance of sialic acids in the recognition process. Reduction of haemagglutination activity by pretreatment of rabbit erythrocytes with trypsin or neuraminidase solutions further supports this view.