Proximal tubular epithelial cells (PTEC) from human renal tissue obtained from biopsy or nephrectomy were grown in monoculture and evaluated in vitro at passage 2–4 for interleukin 6 (IL-6) production in response to medium alone or to interleukin 1 alpha (IL-1α), tumour necrosis factor alpha (TNFα), interleukin 2 (IL-2), interferon gamma (INFγ) or lipopolysaccharide (LPS). IL-6 bioactivity was quantitated using the IL-6-dependent murine hybridoma cell line (B9) and expressed as IL-6 units/ml/10⁵ PTEC. PTEC cell lines exposed to medium alone produced intermediate amounts of IL-6 with substantial variability between cell lines. Introduction of IL-1α resulted in a dose- and time-dependent increase in IL-6 production by PTEC that was maximal at 1 ng/ml IL-1α at 24 h. All PTEC cell lines showed an increased IL-6 production on exposure to IL-1α varying from 1.3- to 24-fold increase over baseline production. This response was completely blocked by anti-rIL-1α. No significant IL-6 production by PTEC could be induced by TNFα, IL-2, IFNγ, or LPS over a broad dosage range. Cycloheximide inhibited IL-6 production without irreversible cell toxicity, indicating de-novo synthesis. IL-6 produced by PTEC had a molecular weight of 26-29 kDa as demonstrated by Western blot analysis. Using PCR analysis we could demonstrate upregulation by IL-1α of IL-6 mRNA in a dose-response fashion, indicating that IL-1α regulates IL-6 production at a pretranslational value of protein synthesis. These results show that human cultured PTEC produce IL-6 under both normal and IL-1α-stimulated conditions, and suggest that they may have a regulatory function in response to cytokines in the setting of inflammation in the renal cortex.