The use of severe combined immunodeficient (SCID) mice to study humoral responses by peripheral blood mononuclear cells (PBMC) from patients with autoimmune haemolytic anaemia (AIHA) was assessed. Upon transfer to SCID mice, PBMC from normal donors and patients with autoimmune thyroid disease (AITD) produced substantial levels of immunoglobulin (Ig), detectable in the plasma of recipient SCID mice. In contrast. the majority of PBMC from AIHA donors did not produce Ig in recipient mice. The capacity of PBMC to reconstitute SCID mice was not related to the donor's age. In one case, remission of AIHA allowed the donor's PBMC to successfully reconstitute SCID mice, despite the fact that the donor had developed immune thrombocytopenic purpura (ITP). AIHA PBMC were viable by dye exclusion and contained cells in various states of activation, as judged by their IgG secretion profiles when culturedin vitro. The proportions of leukocytes in AIHA PBMC (T to B cell ratios, CD4* to CD8* cell ratios and monocytes) were highly variable compared to non-AIHA PBMC. To determine the effect of abnormal lymphocyte proportions on SCID reconsti-tution, depletion experiments were carried out on normal and AITD PBMC. This work demonstrated a requirement for high T cell numbers, especially CD4′cells, and minimal B cell numbers for successful recon-stitution. CD8* depletion of PBMC led to increased levels of Ig production in some instances. It is considered that PBMC from AIHA patients have a defect different from that of other autoimmune disorders, which renders them incapable of reconstituting SCID mice.