We have previously shown that insertional inactivation of tcpA, the gene encoding the major pilin subunit of the toxin-coregulated pilus (TCP), renders Vibrio cholerae O1 strains of El Tor biotype virtually avirulent in the infant mouse cholera model (IMCM). We now report that more refined mutants, bearing an in-frame deletion in tcpA, show a similar dramatic attenuation in vivo. In mixed-infection competition experiments the ratio of wild-type:mutant vibrios increased c. 10(3)-10(5) fold during a period of in vivo growth. An attempt to complement the delta tcpA mutants by providing a functional El Tor tcpA gene in trans was only partially successful. Sera raised against El Tor TcpA were able to passively protect infant mice against challenge with TCP-positive strains of homologous biotype and were also protective against isolates of the novel O139 serovar. These sera failed to protect against challenge with a strain of classical biotype, nor could antibodies to classical TCP confer immunity to El Tor challenge. We conclude that TCP is a critical colonization factor of V. cholerae O1 El Tor and that antibodies to TCP are sufficient to confer protection against such strains in the IMCM. Our data suggest that the biotype-specific epitopes carried by TcpA are of greater vaccine significance than those epitopes common to both proteins.