Abstract Humoral factors in serum of young NZB mice enhance maturation of B-lymphocyte precursors in vitro. A blot ELISA assay identified autoantibodies against the serum factors. NZB-SFs (designated NZB-SF α, p I 3.5 – 4.0, and NZB-SF β, p I 7.8) were purified by sequential steps. Both had a molecular weight (MW) of approximately 23,000 in SDS-PAGE. NZB mice develop autoantibodies against NZB-SFs by 2 months of age; titers increased progressively with age. Non-autoimmune-prone mice did not produce autoantibodies against NZB-SFs. We then developed two hybridoma clones, IIC1C1 and IIC1M4, which produce monoclonal autoantibodies against NZB-SF α and NZB-SF β, respectively. Both IgM autoantibodies could be affinity purified with a column of CNBr-Sepharose 4B gel conjugated with anti-mouse IgM antibody. Neither IIC1C1 nor IIC1M4 abolished bioactivity of recombinant mouse IL-1 α, human IL-1, mouse, rat, or human IL-2, mouse IL-3, or colony-stimulating factor. Neither antibody reacted to recombinant mouse IL-1 α, IL-4, TNF α, or IFN γ in blot ELISA assays. Monoclonal autoantibodies IIC1C1 and IIC1M4 were used to purify NZB-SFs. SDS-PAGE of the affinity-purified NZB-SFs revealed bands of 23 and 60 kDa, and proteins extracted from the bands were reactive to our monoclonal autoantibodies.