Ashton has served a four-match for suspended for unsportsmalike conduct after he was pulled Alesana Tuilagi's hair during the defeat to Leicester.
And although Ashton has been strongly linked with a move to Saracens at the end of the season, the England winger comes straight back into the starting XV for the clash.
There is a rare start for former Sarries wing Noah Cato, too, with Vasily Artemyev and Jamie Elliott making way.
The only other change for the Saints comes in the pack with fit-again Tom Wood coming into the back row for the sidelined Courtney Lawes.
Harlequins will be buoyed by the return of Nick Easter for the trip to Franklin's Gardens.
Easter missed the nail-biting 11-9 victory over Exeter through injury but he has been passed fit to take his place in the back-row - with Tom Guest dropping to the bench.
Lock Olly Kohn is also recalled after returning to full fitness, while Chris Brooker and James Johnston have been added to the front row.
Tomas Vallejos, Tim Fairbrother and Joe Gray all drop to the bench, while Chris York drops out of the squad.
Northampton: 15 Ben Foden, 14 Chris Ashton, 13 George Pisi, 12 James Downey, 11 Noah Cato, 10 Ryan Lamb, 9 Lee Dickson, 1 Soane Tonga'uhia. 2 Dylan Hartley (capt), 3 Paul Doran Jones, 4 Samu Manoa, 5 Mark Sorensen, 6 Phil Dowson, 7 Tom Wood, 8 Roger Wilson.Replacements: 16 Mike Haywood, 17 Alex Waller, 18 Brian Mujati, 19 Christian Day, 20 Ben Nutley, 21 Martin Roberts, 22 Stephen Myler, 23 Tom May.
Harlequins: 15 Mike Brown, 14 Tom Williams, 13 Matt Hopper, 12 Jordan Turner-Hall, 11 Ugo Monye, 10 Rory Clegg, 9 Danny Care, 1 Joe Marler, 2 Chris Brooker, 3 James Johnston, 4 Olly Kohn, 5 George Robson, 6 Chris Robshaw (c), 7 Luke Wallace, 8 Nick EasterReplacements: 16 Joe Gray, 17 Mark Lambert, 18 Tim Fairbrother, 19 Tomas Vallejos, 20 Tom Guest, 21 Richard Bolt, 22 Benjamin Urdapilleta, 23 Sam Smith
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