Isolated carbonic anhydrases CA I and CA II from different species (man, ox, sheep, pig, dog and rat) have been analyzed by immunodiffusion techniques using rabbit specific CA I or CA II antisera. 2. Immunodiffusion shows the absence of cross-reactivity between CA I and CA II isoenzymes. 3. Common antigenic determinants (cross-reactivities) were evidenced between the CA I of the different species under study. 4. To the contrary, cross-reactivity between CA II of the species mentioned above, can clearly be demonstrated only between bovine and sheep CA II.