1. The plasma disappearance rate and the increment in plasma unconjugated bilirubin after intravenous administration of 5.9 mumol of rifamycin SV (RSV)/kg body wt. were investigated in 51 subjects with Gilbert's syndrome and 35 control subjects of both sexes. 2. Both the plasma disappearance rate and the unconjugated hyperbilirubinaemia after RSV administration were higher (P < 0.001) in Gilbert's syndrome. Females, both normal and with the syndrome, showed a significantly shorter t1/2 and a lower hyperbilirubinaemic response as compared with males. A linear correlation (P < 0.001) was present between RSV plasma half-life and the hyperbilirubinaemic response. 3. In vitro, RSV was shown to inhibit sulphobromophthalein (BSP) uptake in rat liver plasma-membrane vesicles with a Ki of 20 mumol/l. Evidence that this effect was due to competition for bilitranslocase was sought on preparations of the purified protein. Under these experimental conditions, RSV inhibited BSP binding with a Ki of 17 mumol/l. 4. Since RSV competes with BSP for binding to bilitranslocase in vitro, the data are interpreted as suggesting that reduced bilitranslocase function might underlie the delayed RSV plasma clearance and the exacerbated unconjugated hyperbilirubinaemia present in Gilbert's syndrome.