The saint is collecting water from a spring, an allusion to his baptising activity in the River Jordan. The words inscribed on the scroll, 'Behold the Lamb of God' (John 1: 29), were spoken by John when Jesus came to be baptised by him.
Formerly attributed to Annibale Carracci, the painting is now thought to be by one of his pupils.
X-radiographs reveal what appears to be a landscape with figures beneath the present composition.
The painting was purchased from the Angerstein collection.