Andres Iniesta's goal a minute before the break has denied Celtic a half-time lead at the Nou Camp.
Giorgos Samaras had shocked the home side in the 17th minute, getting his head to Charlie Mulgrew's launched free kick.
The ball rebounded of Javier Mascherano and past Victor Valdes, sending Neil Lennon into a frenzy on the touchline.
Celtic set about defending that lead and it looked as if they would go into the interval leading.
However, the Spanish ace capped an intricate move, which included Lionel Messi and Xavi, to spare Barca's blushes.
