The Ig kappa L chain synthesized by the mouse hybridoma line F10 forms large fibrils in the lumen of the endoplasmic reticulum. Despite the formation of these inclusion bodies, free kappa-chain is secreted. Both intracellular and secreted kappa-chains in this line migrate faster on SDS gels; thus, the F10 kappa-chain seems to be more compact than normal Ig L chain. In normal kappa-chain, four cysteine residues form intrachain disulfide bonds and the fifth connects the L chain to the H chain. Although the five cysteine residues of the aberrant kappa-chain are in the normal positions, they display an unusual gel pattern when the intrachain disulfide bonds are opened with 2-ME; that is, the intrachain disulfide bonding pattern of F10 kappa-chain seems to be unusual. It is suggested that the abnormal folding pattern favors fibril formation.