Three experiments explored the effects of age, stimulus salience, stimulus luminosity, time lags, and stimulus repetition on negative priming (NP) using unfamiliar novel shapes. No differences in NP were observed between younger and older adults, which counters the view that there is a general decline in inhibitory processes as we age (Hasher & Zacks, 1988). Long-term NP with novel shapes did not occur at either a five-minute or a 24-hour time interval, which challenges reports of long-term NP with once-presented novel shapes (Treisman & DeSchepper, 1996). The finding that NP was more likely to occur with stimulus repetition supported contentions about the importance of stimulus repetition (Strayer & Grison, 1999). Other results demonstrated the sensitivity of the NP effect to minor variations in the experimental conditions. Manipulating the relative salience of target and distractor stimuli increased distractor interference, but did not affect NP. However, reducing the luminosity of prime and probe display stimuli eliminated NP and produced facilitation (positive priming) instead. This facilitation occurred despite the repetition of stimuli, which had proven to enhance NP under different conditions. The present study challenges a number of conclusions about the nature of NP, but also suggests ways of bringing a measure of cohesion to apparently contradictory findings.