We investigated the presence of interleukin 6 (IL6) receptors in human prostate carcinomas and benign prostatic hyperplasias. Interleukin 6 receptor expression was measured at the mRNA level by slot blot analysis using a probe that recognizes mRNA encoding the 80-kDa subunit of the IL6 receptor. Significant expression was found in 29 of 37 (78%) samples of benign prostate hyperplasia (BPH) and in 17 of 17 prostate carcinomas. Quantitative analysis of the expression level revealed that 11 of the 29 positive hyperplasia tissues (38%) and 4 carcinoma samples (23.5%) expressed equal or higher levels of IL6 receptor mRNA than the human hepatoma cell line PLC/PRF5, which contains about 2300 IL6 receptors per cell. We also measured IL6 receptor mRNA levels in three human prostate carcinoma cell lines LNCaP, DU145 and PC3, which are known to contain IL6 receptors because they are sensitive to the cytotoxic action of an IL6-toxin fusion protein. We were not able to detect IL6 receptor expression with the slot blot procedure, but we were able to detect IL6 receptor mRNA using a very sensitive PCR assay. Our data provide evidence that IL6 may play a role in the growth of benign and malignant prostate tumors and suggest that the IL6 receptor could be a target for the delivery of therapeutic agents in prostate cancer.