The aim of this investigation was to determine whether IgG could pass from the blood to the oral cavity. Pure IgG was prepared from monkey serum, by ion exchange chromatography and gel filtration, and was radiolabelled with 125I. This was injected intravenously into eight Rhesus monkeys. Radioactivity could be detected in crevicular fluid washings, and in mixed and parotid saliva samples 30 min after injection. Ultracentrifugation on sucrose density gradients revealed that most of the radioactivity in crevicular fluid washings was associated with proteins having a sedimentation coefficient similar to marker IgG. Radioactivity in parotid saliva was not found in the IgG zone, but was present in zones with sedimentation coefficients of approximately 4·5S and 1S. The results suggest that IgG passes as an intact molecule from plasma to crevicular fluid, and support the hypothesis that serum antibodies could play a role in protection against dental caries.