Administration of human i.v. immunoglobulins was shown to lead to a permanent increase in IgG1 and IgG2 levels in chimpanzees. Half-lives of human IgG1 and IgG2 in chimpanzees were comparable to those found in humans, and no signs of immune elimination were observed. Furthermore, long-term treatment of chimpanzees had no effect on the percentage of immunoregulatory T cells (CD2+, CD4+ and CD8+ T cells) as determined by FACS analysis. In addition, serum IgM levels in chimpanzees were found to be comparable to those in humans, whereas the chimpanzees' IgG levels are slightly elevated due to higher concentrations of IgG2 and, in particular, IgG4.