Bordetella pertussis TOX3201 has a 12-base-pair insertion in the Si subunit gene of pertussis toxin (PTX), which encodes for a 4-amino-acid insertion between residues 107 and 108 of the mature S1 subunit (Black et al., Science 240:656-659, 1988). This mutant strain has been shown to secrete a holotoxin analog of PTX, designated CRM3201, with reduced ADP-ribosyltransferase activity. In the present study, we evaluated the biochemical, biological, and immunoprotective activities of purified CRM3201. Assay of enzymatic activities showed that CRM3201 had 20 to 30% of the ADP-ribosyltransferase activity and 55 to 60% of the NAD glycohydrolase activity of native PIX. CRM3201, however, had only 2 to 6% of the activity of PTX in clustering CHO cells, promoting leukocytosis, inducing histamine sensitization, and potentiating an anaphylactic response to bovine serum albumin. In contrast, activities associated with the B oligomer (binding to fetuin, hemagglutination of goose erythrocytes, and lymphocyte mitogen activity) were comparable to those of native PTX. Injection of BALB/c mice with CRM3201 mixed with Al(OH)3 elicited high titers of antibody to PTX (as measured by enzyme-linked immunosorbent assay), which neutralized a leukocytosis-promoting dose of PTX in these mice and neutralized PTX in a CHO cell assay. Passive transfer of the anti-CRM3201 antibody protected 20-day-old Swiss-Webster mice against a lethal aerosol challenge with B. pertussis 18323. Active immunization with CRM3201 significantly reduced lung colonization in adult BALB/c mice with a B. pertussis