Johnnie is ineligible for Glasgow's remaining Heineken Cup matches so will get game time with Ayr.
A Glasgow Warriors spokesman told glasgowwarriors.org: “We are delighted that Johnnie Beattie has recovered from the shoulder surgery he underwent last summer and that he returned to action against Munster last weekend.
“As our medical team state frequently, recovery from serious injury is not an exact science. For the avoidance of doubt, at the start of the 2010-2011 season, neither Johnnie nor, for example Chris Cusiter, were included in our nominated Heineken Cup squad because of injury.
“We are only permitted to make one change to the squad during the pool stages – and that was made on 2 December when Dave McCall replaced Chauncey O’Toole – therefore Johnnie is not eligible to play in rounds 5 and 6 of the Heineken Cup.
“Johnnie is focusing on performing for Ayr in the British & Irish Cup against Birmingham & Solihull on Sunday – a match that will be watched by a member of the Scotland coaching team – and we wish him and Ayr all the best in that fixture.”
Ayr's assistant coach Peter Laverie told the Ayr.6690102.jp">Scotsman: "I'm sure he would prefer to play in the Heineken Cup this weekend, but the British and Irish Cup will give him decent games and we'll try to exploit his attacking strengths. He's obviously a good player to have, and we are also rooming him in Birmingham with Peter McCallum, an 18-year-old No 8 who is one of our up-and-coming lads, and that will be a great benefit to him and us.
"Johnnie is a great lad and though he must have been disappointed to be told he wasn't eligible, he has been really enthusiastic about playing for us. He was actually worried about putting a boy out of the team, because he's that sort of lad, but we rotate boys in the league and cup anyway."
Laverie added: "This will be his first time playing for Ayr, but Johnnie's a Glasgow boy, who has come up through the ranks with a lot of our guys, and he's a terrific role model."
Another Warrior in British & Irish Cup action is Alex Dunbar who will turn out for Currie against Newport at Malleny Park tonight.
Ayr squad
Forwards: Andy Kelly, Stuart Fenwick, Peter McCallum, Johnnie Beattie (Glasgow Warriors), Robbie Colhoun, Stephen Adair, Dean Stewart, Damien Kelly, Gordon Reid, Paul Burke, Andy Dunlop, Scott Nimmo and Gordon Sykes.
Backs: Gibson Siwo, AJ Macfarlane, Mark Stewart, Ross Samson (Edinburgh), Richard McCallum, Cammy Taylor, Dougie Steele, Robbie Fergusson, Steven Manning and Grant Anderson.