During an outbreak of strangles on a farm with approximately 1500 horses, the spread of Streptococcus equi infection was monitored by repeated nasopharyngeal swabbing and culture. In order to control the infection and prevent new introductions of strangles on to the premises, a system of quarantine and swabbing of cases and all incoming animals was instituted. Long-term carriage of the organism was detected in four clinically healthy convalescent animals, and in two of 350 new ponies; it persisted for between seven and 39 months, but it was detected only intermittently by the culture of swabs which was a much less sensitive method than the culture of guttural pouch lavages taken by endoscopy (45 per cent v 88 per cent sensitivity, respectively, for any single sample). Repeated swabs were often negative for several weeks between positive samples. Nonetheless, in all but one of the long-term carriers, S equi was detected by culture of repeated swabs taken over a period of less than two to three months. Infection was detected unilaterally in the guttural pouches of five of the carriers and was accompanied by large numbers of neutrophils in the lavage samples whether or not there was empyema. Abnormalities of the affected guttural pouches were detectable by radiography but only after the instillation of contrast medium. The study indicated that clinically healthy long-term carriers of S equi present a serious risk of spreading strangles, particularly because they may be detected only by repeated nasopharyngeal swabbing over two to three months.