The membrane characteristics and in vitro function of the circulating B cells from patients with late-onset hypogammaglobulinaemia (CVID) and from patients with sex-linked hypogammaglobulinaemia (XLA) have been investigated using ox red blood cells coated with goat anti-human Ig. All seven patients with XLA had less than 2.0% SmIgM + ve cells and attempts to isolate these cells were unsuccessful. The majority of the 27 patients with CVID had greater than 3.0% SmIgM + ve cells. When cultured either with normal T cells and pokeweed mitogen (PWM) or with Epstein-Barr virus (EBV), isolated SIg + ve cells from eight patients with CVID produced significantly less IgM than normal cells and very little IgG or IgA. They also proliferated less well than normal cells and this correlated with IgM production. Stimulator capacity in the mixed lymphocyte reaction was normal in four out of seven cases. These properties of SIg + ve cells in the blood of patients with CVID are very similar to those of the normal 'immature' B cells found in adult bone marrow.