SummaryIn order to investigate the immunological features of chicken fibrinogen and to compare them with human fibrinogen, monospecific rabbit antichicken and antihuman fibrinogen antisera have been prepared. Both antisera agglutinated human tanned red cells coated with human fibrinogen. Inhibition of the reaction was obtained by prior incubation of rabbit antichicken fibrinogen antiserum with either human or chicken fibrinogen. Using precipitation reactions in agarose gels, precipitating antigen-antibody complexes were obtained between chicken fibrinogen and rabbit antihuman fibrinogen antiserum and vice versa only if the antisera were concentrated or if an increase in the sensitivity of the reaction was achieved using counter Immunoelectrophoresis. Cross absorption of rabbit antichicken fibrinogen antiserum with human fibrinogen did not abolish the precipitating reaction with chicken fibrinogen. In the same way, cross absorption of rabbit antihuman fibrinogen antiserum with chicken fibrinogen did not abolish the precipitating reaction with human fibrinogen.These results indicate that chicken and human fibrinogen share common antigenic determinants; they also indicate additional antigenic determinants in both fibrinogens.