d-Limonene has efficacy in preclinical models of breast cancer, causing > 80% of carcinomas to regress with little host toxicity. We performed a pilot study on healthy human volunteers to identify plasma metabolites of limonene and to assess the toxicity of supradietary quantities of d-limonene. Seven subjects ingested 100 mg/kg limonene in a custard. Blood was drawn at 0 and 24 h for chemistry-panel analysis and at 0, 4, and 24 h for limonene-metabolite analysis. On-line capillary gas chromatography/mass spectrometry (GC/MS) analysis indicated that at least five compounds were present at 4 h that were not present at time zero. Two major peaks were identified as the rat limonene metabolites dihydroperillic acid and perillic acid, and two minor peaks were found to be the respective methyl esters of these acids. A third major peak was identified as limonene-1,2-diol. Limonene was a minor component. At a dose of 100 mg/kg, limonene caused no gradable toxicity. Limonene is metabolized by humans and rats in a similar manner. These observations and the high therapeutic ratio of limonene in the chemotherapy of rodent cancers suggest that limonene may be an efficacious chemotherapeutic agent for human malignancies.