We have previously described a disease of spontaneous but reversible hair loss in a colony of C3H/HeJ mice that clinically and histologically resembles human alopecia areata (AA). As AA in humans is associated with high levels of antibodies to hair follicles (HF), the current study was conducted to determine whether C3H/HeJ mice with hair loss have similar abnormal HF antibodies. Eighteen C3H/HeJ mice with alopecia, 12 unaffected littermates, and 15 control mice were examined for circulating antibodies to HF in C3H/HeJ anagen skin by indirect immunofluorescence and against extracts of isolated C3H/HeJ and human anagen HF by western blotting.Using both procedures, antibodies to anagen HF were present in all C3H/HeJ mice with alopecia but in none of the control mice. The antibodies were also present in most unaffected C3H/HeJ littermates but were absent in mice of an unrelated strain with inflammatory skin disease and alopecia, indicating that their appearance did not result from the hair loss. These antibodies reacted to HF-specific antigens of 40-60 kDa present in murine and human anagen HF. These antigens were also reactive with human AA antibodies. The C3H/HeJ mice and humans with AA, but none of the controls had antibodies to the 44/46 kD antigen, identified as the HF-specific keratins.Thus, C3H/HeJ mice with alopecia have circulating HF autoantibodies similar to those in humans with AA, confirming that these mice are an appropriate model for human AA and supporting the hypothesis that AA results from an abnormal autoimmune response to HF.