The cloning and expression of cytokines and their receptors has led to the development of sensitive and specific immunoassays for their determination in biological fluids such as ascites. Using such assays, the cytokines IL-1-beta, IL-6, IL-8 and TNF-alpha were found in ascitic fluid samples of patients with infected and malignant ascites. IL-6 was found in high concentrations even in ascites samples of not infected hepatic ascites. In addition, high concentrations of soluble TNF receptor p55 and p75 were found in ascites. IL-1 and IL-6 concentrations were increased in infected ascites while concentrations of both TNF receptors were increased in infected and malignant ascites, indicating that the combined measurement of cytokines and cytokine receptors may be helpful for the differential diagnosis of ascites (allowing the differentiation among infected, malignant and uncomplicated hepatic ascites). High concentrations of cytokines and soluble cytokine receptors in ascites, their increase during infection and malignancy, and their high potency to regulate inflammatory processes and other mediators indicate that they play an important role in the pathogenesis of ascites. Thus, measuring cytokines and soluble cytokine receptors in ascitic fluid opens a new window to study the pathogenesis of ascites and eventually may lead to the development of new methods of medical treatment of ascites using specific cytokine antagonists.