We have previously demonstrated the presence of a maltose-inducible major outer membrane protein of 43 kDa in an El Tor Inaba strain of Vibrio cholerae. The occurrence of similar proteins was examined in several isolates of V. cholerae 01. The results indicate that the 43 kDa protein is common to all of the isolates as evidenced by Western blotting analysis with antiserum raised against this protein. The 43 kDa protein was maltose-inducible in most isolates although some strains exhibited a constitutive production of the protein. This protein was present also on V. cholerae 01 organisms harvested directly from the small intestine of rabbits with experimental cholera and it gave rise to specific antibodies after immunization with in vivo grown vibrios.