Abnormal IgA molecule, which lacked in disulfide bridges between α chains, was detected from the sera of patients of monoclonal IgA gammopathy with albuminuria by the method of urea-SDS-PAGE performed in the reducing-agent-free condition. Throughout the procedure, protein in the specimen was kept away from any reducing agent to avoid an artificial production of half molecules during the experiment. A band of 80kDa protein, which showed immunoreactivity to both anti-α- and anti-λ-chain antibodies, was detected in sera from four patients of monoclonal IgA gammopathy accompanied with albuminuria. The molecular mass and the immunoreactivity indicated that the 80kDa protein was so-called“IgA half molecule”(α₁λ₁). The band of α₁λ₁ molecule was faint on the gel in the specimens from patients of no albuminuria. The abnormal α₁λ₁ molecule was eluted from the column of gel permeation chromatography into fractions corresponding to 320kDa when it was performed using urea-free buffer. The chromatographic behavior suggested the non-covalent interaction between the abnormal α₁λ₁ molecules in the non-denaturing buffer condition.