The in vivo bioactivity of polyphenols will depend on their bioavailability. Grape juice is an important source of dietary phenolics. This paper reports results that prove that quercetin (3,3',4',5,7-pentahydroxyflavone) is bioavailable after a single ingestion of red grape juice by healthy volunteers. Blood plasma samples were collected before and after 2 h of ingestion of 100 ml of concentrated grape juice (n = 14), and of a placebo solution (n =6). Significant differences in the variation of the total plasma quercetin content (before and after ingestion) between the grape juice ingestion group (3.1 microg/l increase, as a mean) and the placebo group (6.0 microg/l decrease, as a mean) were found. This relatively low increase in comparison with that obtained after 2 h of ingestion of onions (201 microg/l, as a mean) and with those reported in the literature for other foods/beverages was attributed to differences in the amount of quercetin ingested, in the form in which quercetin is present, and in the food matrix.