Treponema pallidum was investigated for its ability to synthesise glycosaminoglycans or proteoglycans in vitro. Isolated viable T pallidum organisms were incubated with radiolabelled precursors of glycosaminoglycans, sodium 35S-sulphate and 3H-glucosamine (tritiated glucosamine). T pallidum failed to incorporate sodium 35S-sulphate but did incorporate 3H-glucosamine into a macromolecule which may be associated with the surface of the treponeme. This macromolecule was resistant to degradation by specific glycosaminoglycanases. We conclude that T pallidum does not synthesise a capsule containing glycosaminoglycans in vitro.