Abstract Serum-immunoglobulin levels of fiftyfive patients with untreated syphilis were compared with those of thirty-one controls. Both IgM and IgG globulin were raised in primary, secondary, and latent syphilis. IgA was raised at the secondary stage. No associations were found between Venereal Disease Research Laboratory (V.D.R.L.) titres and concentrations of immunoglobulins. The V.D.R.L. titre of most syphilitic sera was not reduced by treatment with mercaptoethanol. Conversely, that of most chronic biological false-positive reactors was reduced by mercaptoethanol. But there were too many exceptions in both groups to make this a reliable diagnostic test.