Interleukin (IL-12) has many effects on the function of natural killer and T cells, and is important in the control of cell-mediated immunity. IL-2 and IL-12 display many similar activities, yet each also induces a distinct set of responses. A human IL-12 receptor subunit has recently been cloned and, like the IL-2R beta and IL-2R gamma, is a member of the hematopoietic receptor superfamily; however, the molecular mechanisms of IL-12 action are unknown. In this report we show that IL-12 and IL-2 induce tyrosine phosphorylation of distinct members of the Janus (JAK) family of protein tyrosine kinases in human T lymphocytes. IL-12, but not IL-2, stimulates the tyrosine phosphorylation of TYK2 and JAK2, whereas JAK1 and JAK3, which are phosphorylated in response to IL-2, are not phosphorylated after IL-12 treatment. The use of distinct but related JAK family tyrosine kinases by IL-12 and IL-2 may provide a biochemical basis for their different biological activities.