: The biological attributes of affective disorders and factors which are able to predict a response to treatment with antidepressants have not been identified sufficiently. A number of biochemical variables in peripheral blood constituents have been tested for this purpose, as a consequence of the lack of availability of human brain tissue. At first, the biological attributes of mental disorders were sought at the level of concentrations of neurotransmitters and their metabolites or precursors. Later on, attention shifted to receptor systems. Since the 1990s, intracellular processes influenced by an illness or its treatment with psychopharmaceuticals have been at the forefront of interest. Interest in biological predictors of treatment with antidepressants has reappeared in recent years, thanks to new laboratory techniques which make it possible to monitor cellular processes associated with the transmission of nerve signals in the brain. These processes can also be studied in plasma and blood elements, especially lymphocytes and platelets. The selection of the qualities to which attention is paid can be derived from today's most widely discussed biochemical hypotheses of affective disorders, especially the monoamine hypothesis and the molecular and cellular theory of depression. Mitochondrial enzymes can also play an important role in the pathophysiology of depression and the effects of antidepressants. In this paper, we sum up the cellular, neurochemical, neuroendocrine, genetic, and neuroimmunological qualities which can be measured in peripheral blood and which appear to be indicators of affective disorders, or parameters which make it possible to predict therapeutic responses to antidepressant administration.