Interleukin-6 is a pleiotropic cytokine and may be a pivotal mediator in the pathogenesis of shock and sepsis, in modulating megakaryocytopoiesis, and in inhibition of tumor growth. Among characteristics of interleukin-6 are regulation of expression of other cytokines, induction of differentiation and proliferation of normal and malignant cells, and inhibition of tumor growth in vivo under experimental conditions. As a major inducer of the acute phase response, interleukin-6 is produced and sets off a chain of events as it acts on effector targets. Preclinical anti-tumor studies with interleukin-6 have provided rationale for probing its role in the therapy of malignancy. The probability is that in the near future interleukin-6 will have established clinical roles as a protein of diagnostic and therapeutic import.