Harlequins have signed 19-year-old second-row forward Stephen Bannister from St Helens.
The former England U17 international made his first-team debut against Catalans in August 2006 and has played for Saints twice this season.
"His first-team chances were limited because of players in his position and the number of youngsters coming through," said a Saints spokesman.
"Therefore, this opportunity was too good for Stephen to turn down."
Harlequins need forward cover following the loss of prop Daniel Heckenberg for the rest of the season with a shoulder problem.
Recent signing Richard Villasanti has failed to shake off a knee injury.