Neonatal rat hybridomas were tested for natural autoantibodies (NAA) production, using different screening procedures. NAA were discovered in 35% of immunoglobulins producing hybridomas. Radioimmunoassay (RIA) on brain and liver homogenates and immunocytochemistry on brain sections are the procedures of choice revealing the major part of the identified NAA. On the contrary, only a small portion of NAA could be detected with indirect immunofluorescence on fixed fibroblasts and with RIA on individual autoantigens. All the NAA revealed proved to be of the IgM type and almost all of them possessed neither organ nor species-specificity. In spite of that, most of the NAA reacted with definite cell populations of nervous tissue such as glia, neurons, ependyma or brain vessel cells. The studied panel of NAA from neonatal rats has common features with similar panels from newborn and old mice, though some species-specific characteristics do exist.