This paper provides an overview of neoplasms which appear to be derived from immunoregulatory cells. Normal T cells play a critical role in the regulation of humoral immune responses by acting as potentiators (helper cells) or inhibitors (suppressor cells) of the B cell transition into immunoglobulin-secreting plasma cells. Certain patients with leukemias or lymphomas of T-cell origin may provide a resource for obtaining homogeneous populations of cells which act as either helper cells (e.g., some patients with the Sézary syndrome) or suppressor cells (e.g., some patients with acute or subacute T-leukemia). The clinical and theoretic implications of such immunoregulatory cell neoplasms are discussed.