Daily local administration at the tumor challenge site of 10 injections of 10 U of recombinant interleukin 2 (IL 2) obtained from different factories induces a consistent, though limited inhibition of the growth of CE-2, a poorly immunogenic methylcholanthrene-induced tumor. By contrast, almost complete tumor inhibition is observed when these injections are performed in mice challenged with tumor cells admixed at 1:5 cell ration with nylon wool column purified lymphocytes obtained from tumor-bearing animals. The host immune system plays a fundamental role in this lymphokine-activated tumor inhibition (LATI), which is derived from the local combination of IL 2 and nonreactive lymphocytes. When the host is sublethally irradiated, or the reactivity of L3T4 and Asialo GM1 lymphocytes is suppressed by in vivo antibody treatment, in fact, LATI no longer takes place. Daily injections of antibody to murine interferon-gamma or cyclosporin A have the same effect, indicating that lymphokine release plays an important role in the recruitment of host reactivity. The morphological data show that when LATI is taking place the tumor challenge area becomes infiltrated by mononuclear cells and granulocytes (mostly eosinophils), which establish close contacts with each other and with tumor cells as determined at ultrastructural analysis. Tumor draining lymph nodes display marked expansion of cortical and paracortical areas. Lymphocyte proliferation, interferon-gamma release and cytotoxicity against CE-2 and YAC-1 target cells are greatly enhanced during LATI. Contralateral lymph nodes and the spleen also show a slight increment of these functions. In mice challenged with CE-2 tumor cells only and receiving daily IL 2 injections, these reaction functions (with the exception of interferon-gamma secretion) are also enhanced, though to a lesser extent than during LATI and only in tumor-draining lymph nodes. Last, the growth of a second contralateral tumor challenge is significantly impaired during or after LATI, showing that a persistent and effective systemic reactivity can be quickly induced in this way.