tox·in-an·ti·tox·in (t?k?s?n-?n?t?-t?k?s?n)n.1. A pair of genes that encode a stable, toxic protein and a corresponding unstable antitoxin that is either a protein or RNA, found in many bacteria either on a plasmid or on the bacterial chromosome.2. A mixture of a toxin and its antitoxin with a slight excess of toxin, formerly used as a vaccine.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.toxin-antitoxin n (Pharmacology) a mixture of a specific toxin and antitoxin. The diphtheria toxin-antitoxin was formerly used in the US for active immunization Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014