The M-protein genes of Streptococcus equi isolated from 17 outwardly healthy horses after 4 strangles outbreaks had ended, including a quarantined animal, were compared with those of 5. equi isolates from 167 active cases of strangles across 4 countries. The healthy horses included 16 persistent 5. equi carriers, at least one from each of the four outbreaks. These carriers, despite being outwardly healthy, had empyema of the guttural pouch(es), an enlargement of the equine Eustachian tube. A persistent carrier from two of these outbreaks, the quarantined animal and a healthy animal with normal guttural pouches, from which 5. equi was isolated only once, were colonized by variant 5, equi with truncated M-protein genes (24% of outwardly healthy animals with 5. equi). The truncated M-protein genes had in-frame deletions in slightly different positions between the signal sequence and the central repeat region, equivalent to approximately 20% of the mature expressed protein. Immunoblotting with antibody to recombinant M-protein confirmed that the variants expressed a truncated form of the M-protein. In contrast to the outwardly healthy 5. equi carriers, only 1/167 of 5. equi isolates from strangles cases possessed a truncated M-protein gene (<1%; Fisher's exact test, P = 0.0002). Compared with isolates from healthy horses with a truncated WI-protein, much more of the N terminus of the truncated M-protein was retained in the variant 5. equi from a strangles case. Variant 5 equi from outwardly healthy animals were more susceptible to phagocytosis by neutrophils in vitro than typical isolates. This is the first report of detection of 5, equi with a truncated M-protein. The distribution of the variants between strangles cases and carriers suggests that the 80% of the M-protein retained in the variants may contribute to colonization whilst the deleted portion of the gene may be needed for full virulence.