The mutagenic activity of red wine and grape juice extracts was investigated using the Ames Salmonella microsome assay system and compared to the mutagenicity results of urine from individuals who consumed red wine or grape juice according to the same test. All subjects participating in the study were healthy nonsmokers. All subjects refrained from fried foods, alcoholic beverages, and grape juice for 24 hours prior to the study. One group of subjects was then put on a liquid diet for 12 hours which included lowfat white milk, fruit and vegetable juices, soft drinks, water, and sugar. A 6 hour control sample was obtained during the last 6 hours, and the subjects were given either 750 milliliters (ml) of red wine or 1180ml of grape juice at the end of the 12 hour period. A second group of individuals were permitted a regular diet after the initial 24 hour period with consumption of the wine or grape juice after collection of the 6 hour control sample. The bacterial strains used for the assay included (TA-98) and (TA-100) with and without S9 mix. Red wine and grape juice extracts obtained from an Amberlite XAD2 resin column showed toxic and mutagenic activities upon testing with (TA-98). Microsomal activation was not required for activity, but increased responses were observed in its presence. Urine samples spiked with the grape juice and red wine extracts also showed toxic and mutagenic activity with induction of 50 percent more revertant colonies than the corresponding stock solution. Urine samples obtained from individuals on either the liquid diet or a regular diet did not show any increase in mutagenic or toxic activity after ingestion of the red wine or grape juice, even with inclusion of beta-glucuronidase in the assay. The authors conclude that the ingestion of red wine and grape juice will not interfere with the mutagenicity testing of urine samples.