Fifty-six inpatients with unipolar depression completed treatment with isocarboxazid. In comparing the differences between responders and nonresponders, it was found that psychomotor retardation, pathological guilt, daily persistence of unremitting symptoms, phobic anxiety, dexamethasone suppression test nonsuppression, and neuroticism were significantly more common among nonresponders. Reactivity of mood, blaming others, and extraversion were more common in responders. Total endogenous depression scores on the Newcastle 1, Newcastle 2, and Michigan scales were also significantly higher in nonresponders. Attained platelet monoamine oxidase inhibition was similar in both groups.