Experimental autoimmune thyroiditis (EAT) is an autoimmune disorder of the thyroid gland induced in susceptible strains of mice by thyroglobulin (Tg). We recently showed that low Mr (<10 kDa) Tg tryptic fragments and a 40 amino-acid peptide (F40D) from Tg could induce EAT as well as native Tg. Because it has been reported that autoantibodies (A-Abs) express VH families preferentially located in the D-proximal VH gene segment, we investigated whether A-Abs specific for one pathogenic peptide from Tg were also skewed towards D proximal VH gene segment.In that respecr. we immunized CBA/J mice with EAT inducer antigens of decreasing sizes: Tg (660 Mr),<10 kDa Tg trypic fragments or F40D peptide (4.9 kDa Mr) from Tg. The VH gene segments utilized by immune spleen cells were determined by hybridization to total spleen cell RNA previously deposited onto nylon membranes and densitometric scans. This study was conducted on days 7 and 9 after determination of the maximum amounts of mRNA coding for immunoglobulins and on day 28 when A-Ab levels are the highest. Results were compared to VH gene segment expression both in normal and adjuvant-injected mice.We found that immunization of CBA/J mice with EAT inducer antigens stimulate B cells the restriction of which, in terms of VH family usage, depends on the size of the immunizing antigen: the larger the antigen, the higher the numbers of VH families used. Moreover, we found that B cell stimulation consecutive to immunization with the peptidic antigen inducing EAT occurs in VH 452 family. a VH encoded by D-proximal gene segment.