We have probed for the presence of interleukin 6 (IL6) receptors in prostatic carcinoma cell lines (LNCaP, DU 145, and PC3) by examining their sensitivity to the cytotoxic effects of a chimeric toxin composed of IL6 and Pseudomonas exotoxin (PE). All three cell lines were killed by IL6-PE66(4)Glu, a version of IL6-PE in which the binding domain of native PE has been mutated to debilitate PE binding to its own receptor. This cytotoxic activity confirmed the presence of IL6 receptors on prostatic carcinoma cells. We have measured the number of IL6 receptors found on these cells and have further determined that they secrete IL6. These data provide evidence that IL6 and its receptor may play an important role in human prostate cancer.