This paper reviews the effectiveness of local interleukin-2 (IL-2) therapy and its effector induction mechanisms in vivo. Local therapy with IL-2 and/or lymphokine-activated killer (LAK) cells is associated with far fewer side effects than systemic treatment. Local infusions of IL-2 into malignant pleural effusions and the peritoneal cavity induce LAK cells and secondary production of other cytokines responsible for up- or down-regulation of LAK activity. An understanding of the regulatory mechanism of local LAK induction in vivo may provide the rationale for a more effective therapeutic modality for cancer in humans.