Ageing and depression have been found to be associated with poorer performance in mnemonic discrimination. In the current study, a two-response format mnemonic similarity test, Cognitive Drug Research MST, was used to compare these effects. Seventy-six participants were tested; with 52 participants in the young group, aged 18¿35 years, and 24 participants in the elderly group, aged 55 years or older. Twenty-two young participants and 10 elderly participants met DSM-IV criteria for MDD or dysthymia. Age-related deficits were found for lure identification and speed of response. Differences in speed of responses to lure images were found for younger depressed participants, and depressive symptom severity was found to be negatively associated with lure identification accuracy in the elderly. These findings may be viewed as putative behavioral correlates of decreased pattern separation ability, which may be indicative of altered hippocampal neurogenesis in aging and depression.