Celtic failed to score for a second pre-season game, losing 1-0 to German third division side Stuttgart Kickers.
Neil Lennon's experimental first-half line-up conceded a headed goal just before the break and they struggled to carve out any second-half openings.
Their hosts had several chances to add to the lead in the final 20 minutes.
Celtic, who opened their tour with a goalless draw against Augsburg in midweek, started with a midfield diamond with Paddy McCourt playing in a deep central role. The Northern Ireland international moved to the left wing in the second half when Celtic played a more square midfield four after the introduction of James Forrest, Beram Kayal and Kelvin Wilson.