Determining the identity of cotton fiber biopolymers, how these wall biopolymers are organized, and the linkages that are formed to create a structurally sound wall are central to our interest of enhancing cotton fiber length and strength. Two classes of wall-associated proteins have been characterized from cotton fiber primary cell walls. One protein class is closely associated with the carbohydrate polymers as a structural component. The other class is defined as an ionically bound class since these proteins are eluted from wall preparations with dilute salt solutions. The presence of the modified amino acid, hydroxyproline, in hydrolyzates of fiber walls suggests that the structural protein in fiber is similar to proteins characterized from other higher plants. Isoditryosine, the putative cross-linking dipeptide, is present at a fairly-constant level throughout development when expressed as a percentage of all amino acids hydrolyzed from purified cell wall preparations. Formation of isoditryosine is catalyzed by the enzyme peroxidase. Peroxidase is in the class of ionically-associated fiber cell wall proteins. The amount of peroxidase activity present in fiber cell walls has been measured using several artificial electron donors. Peroxidase protein levels in developing fibers were quantified using an immuno-slot blot assay.