Members of Air Berlin and Air Serbia's loyalty schemes can now earn and redeem award miles when flying with the other airline.
The agreement means Air Berlin Topbonus members can take advantage of frequent flyer benefits across Air Serbia's entire network, and vice versa for Air Serbia Etihad Guest members.
Members of both programmes can also take advantage of benefits including extra baggage allowance, priority check-in, lounge access and pre-boarding.
And, until September 11, Topbonus and Air Serbia Etihad Guest members will receive double award miles when travelling with the other carrier.
Etihad Airways acquired 49 per cent of Air Serbia in March last year (see news, March 2014).