As several of the biological functions of NK cells are similar to the biological effects of IL-6, we tested for the production of this cytokine by cultured NK cells. Conditioned medium from the NK cells supported the proliferation of an IL-6 dependent human leukemic plasma cell line in dose-dependent fashion and this response was abolished by a neutralizing anti-IL-6 serum. Analysis of the mRNA from the NK cell cultures by RNA blotting demonstrated a specific transcript for IL-6 providing further confirmation that these cells elaborate this factor in culture.