The effects of consumption of glucosinolate-containing Brussels sprouts on plasma and urinary glutathione S-transferase (GST) class-α and -π were investigated. Five male and five female non-smoking volunteers were randomly assigned to two groups in a crossover design. Five persons started on a glucosinolate-free diet (control period), while the other five consumed 300 g of cooked Brussels sprouts per day, at the expense of 300 g of glucosinolate-free vegetables (sprouts period). Dietary regimes were reversed after 1 week, GST levels were measured by enzyme-linked immunoabsorbent assay. At the end of the sprouts period, a significant increase (1.5-fold) in plasma class-α GST levels was observed in males but not in females (control versus sprouts, paired t-test; P-values 0.031 and 0.317 respectively), while plasma GST class-π levels as well as secretion of urinary GST class-α and -π levels remained unchanged. We conclude that (i) increased plasma GST class-α levels in males originate probably solely from the liver and not from stomach, intestine or kidney; (ii) males are more susceptible for induction of hepatic GSTs than females; and (iii) urinary GST concentration seems less useful as a biomarker for hepatic GST induction.