antibodies from theresulting lines and clones were examined for their binding to a variety of auto‐antigens and micro‐organisms by ELISA and fluorescence assays. Auto‐antigens tested included Fc of IgG, ssDNA and dsDNA, cardiolipin, histones 1‐4, collagens type I and II. thyroglobulin, cytoskeletal components, and a tissue section screen. Of 71 cell lines tested, all but 19 showed some autoreactivity. All 32 fetal liver lines reacted to some seif‐antigens. In cord blood clones, 16 out of 26 bound to auto‐antigens. Many of the clones reacted with more than one auto‐antigen and were ‘polyrcactive’. Some of the cord blood clones bound to extracts of micro‐organisms, showing specificity for both endogenous and exogenous antigens. The high frequency of CD5+ B cells in the cord blood (>50%) and fetal liver (>70%) argues for many of these clones being derived from this subset. Therefore, our data support the concept that many ‘early’ B cells produce polyreactive IgM which can bind to a variety of different auto‐antigens and microorganisms. These IgM antibodies are similar to those described by other