We prove that any positive power bounded operator T in a KB-space E which satisfieswhere B E is the unit ball of E, g ∈ E + , and 0 η < 1, is mean ergodic and its fixed space Fix (T ) is finite dimensional. This generalizes the main result of [E.Yu. Emelyanov, M.P.H. Wolff, Mean lower bounds for Markov operators, Ann. Polon. Math. 83 (2004) 11-19].Moreover, under the assumption that E is a σ -Dedekind complete Banach lattice, we prove that if, for any positive power bounded operator T , the condition (1) implies that T is mean ergodic then E is a KB-space.