Previous attempts to establish a relationship between impaired cell-mediated immunity (CMI) and major mood disorders have been limited by a failure to explore the relevance of depressive subcategories or to assess CMI by in vivo methods. In this case-control study CMI was assessed in 57 patients with major depression (31 with melancholic, 26 with non-melancholic disorders), and in age- and sex-matched controls by both in vitro and in vivo immunological techniques. Compared with control subjects and patients with non-melancholic depression, patients with melancholia demonstrated reduced in vivo CMI as assessed by delayed-type hypersensitivity (DTH) skin responses. Although increasing age, severity of depression, hospital admission for treatment, and reported weight loss are correlates of melancholia, none of these factors alone, or in combination, accounted for the differences in DTH responses observed between the two depressive subtypes. These data suggest that impaired CMI in vivo may be limited to those with melancholic disorders. At this stage the factors which account for this effect are unclear.