A completely synthetic gene encoding fragment C, a approximately 50-kDa fragment, of botulinum neurotoxin serotype A was constructed from oligonucleotides. The gene was expressed in Escherichia coli, and full-sized product was produced as judged by Western blot (immunoblot) analysis. Crude extracts of E. coli expressing the gene were used to vaccinate mice and evaluate their survival against challenge with active toxin. Mice given three subcutaneous vaccinations were protected against an intraperitoneal administration of 10(6) 50% lethal doses (ID50) of serotype A toxin. The same mice survived when challenged with 3 LD50 of botulinum toxin serotype E but died when challenged with 10 LD50 of serotype E or 3 LD50 of serotype B. Purified fragment C was compared with the botulinum toxoid vaccine in a vaccination and challenge study. Fragment C was as efficacious in protecting against challenge with active botulinum neurotoxin serotype A as the toxoid vaccine. This recombinant protein product has many properties that make it a good candidate for human use to protect against botulinum toxin.