Low molecular weight phenylpropanoid components of plant cell walls, ingested by humans as dietary fruit and vegetables, are released from covalent linkages to the cell wall by bacterial fermentation in the lower gut.  Based on their structural characteristics, such phenylpropanoids and their microbial metabolites potentially have the capacity to interact specifically with the cyclooxgenase (COX) enzymes, important mediators of the inflammation associated with developing neoplasia.  These cell wall components could therefore constitute an important dietary source of potentially bioactive compounds. A combination of enzymatic and chemical cell wall hydrolysis was used to liberate covalently bound low molecular weight phenylpropanoid derivatives from the polymer matrix of fruit and vegetable cell walls.  A range of ester- and ether-linked hydroxycinnamic and benzoic acids and derivatives, including significant quantities of dimeric 4-hydroxy-3-methoxycinnamates, was recovered from all samples examined.  An estimate of the bioavailability of these cell wall components was made by quantifying the proportion of measured cell wall phenylpropanoids released from the wall by enzyme digestion.  In approximately one half of the plant foods examined, these cell wall components were shown to occur at levels which represent a significant dietary burden (based on measured bioavailable compounds in a serving of 125 g plant fresh weight), and thus their presence in the diet is generally underestimated. The major cell wall associated hydroxycinnamates and the 4-hydroxy-3-methoxycinnamate dehydrodimers, as well as their major microbial metabolic products the phenylpropanoates, were examined for COX - inhibitory activity using an enzyme immunoassay.  Results showed a broad spectrum of activities, mainly non-specific.  Cell wall phenylpropanoid derivatives thus have the potential to alter the pattern of prostanoid formation in individuals consuming, as part of their diet, food plants with a high cell wall hydroxycinnamate content.