Rhesus monkey IgG, IgA and IgM were purified from pooled normal serum and labelled with 125I. Four monkeys were injected intravenously with approximately 50 microgram of each purified immunoglobulin containing between 25 muCi and 50 muCi of 125I. Iodination was approximately 1 atom per 14 molecules of IgG and IgA and per 2.5 molecules of IgM. Samples of serum were taken for up to 22 days after injection and radioactivity compared with that in a sample taken within 30 min of injection. Sucrose density ultracentrifugation of serum samples revealed that in animals given IgG or IgA, radioactivity was associated with 7S protein peaks, and in animals given IgM radioactivity was associated predominantly with a 19S protein peak. Most of the radioactivity could be precipitated from serum samples with the appropriate monospecific antiserum. The mean half-life of rhesus monkey IgG was estimated as 8.3 days, IgA as 4.5 days and IgM as 4.7 days.