Immunodeficient C.B-17 scid/scid (SCID) mice were reconstituted with human peripheral blood mononuclear cells and analyzed for humoral immunity. The majority of adoptively transferred animals had serum levels of 1-4 mg/ml of human IgG 8-12 weeks after i.p. reconstitution with 20 x 10(6) PBMC, whereas the IgM and especially IgA concentrations were considerably lower. The half-lives of human IgG, IgM, and IgA in SCID mice were 12 days, 36, and 23 hours, respectively. Furthermore, IgA was rapidly secreted into bile indicating that the low IgA concentration was mainly caused by a high turnover rate. IgG subclass distribution in mouse serum was similar to that found in the donor serum. Irradiation with 3 Gy prior to adoptive transfer resulted in increased levels of human IgG early after reconstitution, whereas both IgM and IgA concentrations were impaired. A polyclonal serum Ig pattern was found 4 weeks after transfer of human cells later frequently followed by a predominance of oligoclonal bands. Unexpectedly, oligoclonal bands were also found using donors with a negative Epstein-Barr virus serology. Human cells were found to reside in the peritoneal cavity for several months. Within 2 weeks of reconstitution, human cells were also identified in lymphoid structures in the vicinity of the liver hilus with a later spread to other lymphoid organs. Homing of human cells to skin and gut was not seen.