Mirror generalization for animals and objects but not buildings is delayed by the inhibition of mirror generalization for reversible letters (e.g., b/d). We behaviorally investigated whether this inhibition occurs at a perceptual stage of processing or at a higher associative stage. 50 adult readers performed a negative priming experiment in which they had to determine whether two letters then two anonymous faces are identical or different. Participants were slower to determine that two anonymous faces facing opposite orientations were identical when preceded by a reversible letter and its mirror-image counterpart (e.g., b/d) than two non-reversible letters (e.g., a/h). This result replicates and extends to hardly nameable probes previous results that demonstrated a similar negative priming effect with nameable probes (i.e., animals and objects). These results suggest that mirror generalization for letters and faces and the inhibition of mirror generalization for reversible letters occur at a perceptual stage of processing.