Hybridomas were constructed from splenic B cells after mitogen stimulation in vitro with lipopolysaccharide and dextran sulfate for 9 to 11 days. Extensive proliferation and differentiation (secretion of IgG isotypes) was evident in these cultures before fusion. Hybridomas that express a VH gene segment whose germ-line sequence is known were isolated, and the nucleotide sequences of these expressed VH genes were determined. A total of 3775 VH nucleotides was analyzed in this way, and only one difference from the germ-line VH sequence was observed. The rate of V gene somatic mutation that has been estimated to occur during antigen-driven immune responses in vivo is 10(-3)/base pair/cell division. Given an estimated value for the number of cell divisions that occurred before hybridoma formation, at least 15 changes from the germ-line VH sequence should have been observed if mutation had been occurring at the in vivo rate during the culture period. Therefore, the data suggest that mitogen-driven B cell proliferation and differentiation are not sufficient to induce the hypermutation of Ig V region genes.