An intact gene coding for wild type diphtheria toxin was reconstituted in vitro from fragments of cloned inactive diphtheria toxin mutants with defects in different parts of the gene. The reconstituted DNA template was amplified using the Taq DNA polymerase chain reaction, providing a virtually unlimited supply. The toxin was expressed in vitro by transcription from a T3 RNA polymerase promoter, followed by translation of the mRNA in a rabbit reticulocyte lysate system. Diphtheria toxin synthesised in this manner was shown to be as toxic as natural toxin and behaved like natural toxin in every way tested. This can serve as a general approach for study of toxic proteins, opening up the possibility of in vitro gene manipulation. Reconstitution of the wild type gene from fragments followed by polymerase chain reaction amplification bypasses the hazards of cloning protein toxins.