In this study, we have measured the following biological variables in 78 depressed inpatients: adrenocorticotrophic hormone (ACTH) responses to corticotropin releasing factor (CRH: 100 micrograms intravenously), postdexamethasone cortisol and ACTH values, and circulating concentrations of L-tryptophan (L-TRP). Patients were categorized according to the DMS-III as (1) minor depression, (2) simple major depression, and (3) major depression with melancholia/psychotic features. By means of various pattern recognition methods, we determined whether these diagnostic groups constitute discrete biological classes or form relevant stages (i.e., continuous categories) in a continuum of progressing biological dysfunction. We established that unipolar depression constitutes one biological continuum characterized by a progression of lower CRH-induced ACTH responses, lower L-TRP levels, and higher postdexamethasone cortisol and ACTH values along the diagnostic spectrum. However, the biological differences in these markers between melancholia and minor depression are quantitatively prominent to the extent that they become qualitative. These findings support the biological heterogeneity hypothesis of melancholia. Simple major depression is a heterogeneous class with regard to the biological markers employed.