Frequencies of B cell clonal precursors producing antibodies that react with mouse thyroglobulin, mouse erythrocytes, beef hemoglobin, KLH, and sheep erythrocytes were determined by limiting dilution analyses among small, resting lymphocytes, and among large activated cells from normal adult mice. While frequencies of clones reacting with external antigens were equally distributed in large and small B cells, most, if not all, autoreactive B lymphocytes were found in the large cell fraction. Analysis of antithyroglobulin hybridomas isolated from normal mice revealed dissociation constants ranging from 10(-6) to 5-6 X 10(-7). Treatment of normal donors with antimitotic drugs dramatically decreases the frequencies of autoreactive B cells, but not those of B lymphocytes reacting with external antigenic molecules. Taken together, these experiments show that immunocompetent, autoreactive B lymphocytes are activated and cycling cells in the peripheral lymphoid tissues of normal individuals.