IgM-FTA tests have been carried out on 209 sera from 169 patients with treated or untreated syphilis at various stages and on 128 sera from 109 patients, born in areas where yaws is or was prevalent, with treated or untreated latent treponemal disease. IgM anti-treponemal antibody was found in virtually all cases of untreated early or early latent syphilis but in only 23 per cent. of sera from patients with untreated late latent syphilis. After treatment the antibody usually disappeared within one year, but it persisted in a minority of patients, including some treated for late symptomatic or congenital syphilis. Except in isolated cases there was no clinical evidence to suggest continued disease activity, although a third of the patient in whom the antibody persisted for more than 2 years after treatment were noted to be homosexuals. The test may assist in differentiating untreated early latent from late latent syphilis.