The purpose of this study was to determine whether quantitative or qualitative factors are of major importance in the destruction of red cells sensitized with incomplete warm autoantibodies of subclass IgG1. To that end, the relative amount of igG1 antibody present on the red cells of patients with autoantibodies of this subclass only, was measured by means of continuous flow cytofluorometry. This method appeared to give an idea of the amount of antibody on red cells and was reproducible. The intensity of the fluorescence of patient's red cells, measured after incubation with a FITC-labelled anti-IgG1, was compared with the presence or absence of signs of increased haemolysis in vivo and the cytotoxic activity of normal monocytes towards these red cells in vitro. It appearedthat it was predominantly the amount of IgG1 autoantibody that determined whether or not these antibodies induced haemolysis in vivo or cytotoxicity of monocytes in vitro. This was also true with methyldopa-induced IgG1 autoantibodies.