Thirty-three dogs were categorised according to their clinical signs of flea allergy dermatitis and reactivity to feeding fleas (Ctenocephalides felis felis). A soluble extract of whole fleas (FS), fractions of this extract separated by chromatography, and a commercially available flea antigen extract were used in intradermal skin tests (IDST) to establish the presence or absence of type I and type IV hypersensitivity. The reactions were measured and the results were analysed using three grading systems commonly reported in the literature. The results of the IDST for the groups of dogs varied according to the grading system used. FS, the most effective of the antigen preparations, identified 94 per cent of dogs which reacted to feeding fleas when a result was considered positive if the mean diameter of the wheal at the antigen injection site exceeded the diameter of the wheal at the negative control site by five mm at 15 and/or 30 minutes after injection.