p-Coumaric acid is one of the predominant phenolic acids acylating the cell walls of grasses; p-coumarates are mainly esterified by lignins and arabinoxylans. Here we describe the production and characterisation of two monoclonal antibodies against p-coumarates. The 5-O-pCou-Ara(1-->4)Xyl was chemically synthesized and conjugated to a carrier protein. Two interesting antibodies were obtained, hereinafter named INRA-COU1 and INRA-COU2. The specificity of these monoclonal antibodies has been evaluated using competitive-inhibition assays with different oligosaccharides and phenolic compounds. INRA-COU1, recognized free p-coumaric acid or p-coumarate esters. INRA-COU1 did not react with any of the other hydroxycinnamic acids and related compounds found in plants. INRA-COU2, only recognizes esterified p-coumarate. These antibodies were used to study the localization of p-coumarates in the cell walls of grasses. Immunocytochemical analyses indicated noticeable amounts of p-coumarate in the cell walls of the aleurone layer of wheat grain, in the epiderm of cereal straw, and in the exoderm of wheat root. The use of these antibodies will contribute to a better understanding of the organisation and developmental dynamics of cell walls in Graminaceae.