The metalloantibiotic bacitracin is a known inhibitor of protein disulfide isomerase (PDI). The disulfide-linked interleukin-12 (IL-12) alphabeta-heterodimer and beta2-homodimer forms are crucial mediators of cell-mediated immune responses and inflammatory reactions. Bacitracin was found to potently block secretion of both the alphabeta- and beta2-dimer forms of IL-12, while it did not affect secretion of the beta-monomer. This inhibition coincided with a reduction in the intracellular amount of PDI found in complex with the beta-chain during intracellular transit. Bacitracin did not affect mRNA levels of the alphabeta- and beta-chain. Similar to bacitracin, N-acetylcysteine blocked alphabeta- and beta2-secretion as well as PDI-beta-chain complex formation. Thus, blocking PDI or shifting the endoplasmic reticulum towards a more reduced status disrupts the oxidative folding pathway or assembly of IL-12 dimer forms. The assembly stage of cytokines in the endoplasmic reticulum may represent a novel target for pharmacological intervention.