A vaccine utilizing purified mutant alpha toxins from Clostridium septicum for humans and animals against infections caused by C septicum. Persons potentially affected by C. septicum infections include colonic cancer patients, diabetics, leukemia patients, and neutropenics. The alpha toxin mutant of the vaccine lacks the toxicity of a native C. septicum alpha toxin. A serum comprising antibodies raised to the alpha toxin mutant is also available for treating humans or animals against C. septicum infections. The serum may be used in a method for conferring passive immunity against C. septicum. Antibodies to the alpha toxin mutant may be used in diagnostic tests or in treatments to clear alpha toxin from bodily fluids. The mutant alpha toxin may be produced by recombinant methods using cDNA encoding the toxin, the cDNA contained for example in a plasmid or host cell.