Recombinant (r) human IL-2 was administered in vivo to improve homing and engraftment of rheumatoid arthritis (RA) patients' peripheral blood mononuclear cells (PBMC) into severe combined immunodeficient (SCID) mice. Human rIL-2 treatment resulted in augmented human Ig production and induced IgM rheumatoid factor (RF) of human origin in SCID-RA chimeras. The increment of human serum IgG in SCID-RA chimeras after IL-2 treatment ranged between 15 and 43% and for IgM between 50 and 98% during 2-8 weeks postengraftment. Human IgM-RF was detectable after 1 to 2 weeks after engraftment and persisted over a period of 10-13 weeks. No RF was produced in SCID mice engrafted with PBMC from healthy individuals with or without exogenous rIL-2 administration. Thus, human rIL-2 expanded autoreactive clones involved in the production of RF in the SCID-RA chimeras. The present study provides a novel approach to establish an in vivo SCID-RA model to study the cellular and molecular mechanisms involved in the production of RF and development of a RA-like lesion.