Starting from unimmunized BALB/c splenocytes, B-cell clones were obtained by lipopolysaccharide (LPS) stimulation, and the frequencies of their anti-TNP, anti-BALB/c actin, and anti-BALB/c tubulin secretion were determined. The culture conditions were then chosen so as to have one anti-TNP precursor per positive well. Out of the 41 wells containing one anti-TNP antibody-secreting cell, nine (22%) also reacted either with actin or with tubulin and five (12%) with both actin and tubulin. Using horse red blood cells to which trinitrophenyl (TNP) had been coupled, spleen cell rosettes were prepared, enriched, micromanipulated, and cultured individually. Of the 500 micromanipulated and cultured TNP antigen-binding cells, 28 were found to secrete antibodies directed against TNP. Eight of these 28 clones (28%) also reacted with either actin or tubulin, and five (17%) reacted with both actin and tubulin. The frequency of these multispecific clones is of the same order of magnitude as that obtained with the limiting dilution experiments. The results show that unimmunized BALB/c mouse spleen contains precursor cells that secrete natural multispecific autoantibodies.