Parathyroid hormone (PTH) is the main hormone controlling calcium concentration in the extracellular fluid (ECF) through its biological activity on bone, kidney and intestine. However, data published over the last two decades indicate that PTH may act as an immunomodulator. The purpose of the present review is to summarize the effects of PTH on various immune functions. Polymorphonuclear leukocytes of patients with chronic renal failure (CRF) and elevated blood levels of PTH showed impaired migration, reduced phagocytic and bactericidal activity, and inhibited granulocyte chemotaxis. Antibody production and T and B lymphocyte proliferation are affected by PTH, both in vivo and in vitro. Possible implications of the involvement of PTH and its fragments in CRF are discussed.