The mechanism of uveitis following cataract surgery, which is mediated in part by the cyclooxygenase pathway, is complex, the complexity of which is likely to be due to several factors. We investigated the possible local involvement of interleukin 6 (IL-6) in this response. Using a specific bioassay, we showed a dramatic increase of IL-6 levels (> 4 x 10(3) times) in all of the aqueous humors of 12 patients following cataract surgery. The IL-6 levels in serum samples were below detection limits, indicating a local production of this cytokine. In addition, the injection of highly purified recombinant interleukin 6 in the anterior chamber of the eye in rabbits resulted in an inflammatory response. These observations suggest that IL-6 may play a crucial role in the occurrence of inflammation after cataract surgery, and it may lead to new therapeutic approaches to this problem.