toxoid,  bacterial poison (toxin) that is no longer active but retains the property of combining with or stimulating the formation of antibodies. In many bacterial diseases the bacteria itself remains sequestered in one part of the body but produces a poison (exotoxin) that causes the disease manifestations. Heating this poison or treating the poison with chemicals converts the exotoxin into a harmless toxoid; when the toxoid is injected into the body, it causes the formation of antibodies that will also react with the active toxin and thus confer immunity from subsequent infection.

Toxoids are used extensively in the production of vaccines, the most prominent examples being the toxoids of diphtheria and tetanus, which are often given in a combined vaccine. Toxoids used in modern vaccines are commonly obtained by incubating toxins with formaldehyde at 37° C (98.6° F) for several weeks.