With the decline in incidence of diphtheria in Europe and the USA, many laboratories no longer routinely culture throat swabs for Corynebacterium diphtheriae. However, there is an outbreak of infection with toxigenic strains in Russia and most adults do not have protective levels of antibody. Non-toxigenic strains are known to cause local disease and lysogenic conversion probably occurs in vivo as well as in vitro. Non-toxigenic C. diphtheriae var. gravis, formerly quite rare, has been isolated with increasing frequency in the UK over the last five years. During prospective screening at one Sexually Transmitted Disease Clinic, six (1%) of 578 homosexual men were found to harbour the organism in the throat, four of them with clinical pharyngitis. Only one of 1696 heterosexual men and women were found to be carriers. Seven cases of endocarditis due to this organism were reported in a single year in Sydney, Australia and non-toxigenic C. diphtheriae var. mitis has caused four cases of endocarditis in Switzerland. Non-toxigenic strains are responsible for pharyngitis and occasional invasive disease and should be treated. Routine screening of throat swabs should not be abandoned.