Monocyte monolayer assays (MMA) were performed using red cells (RBCs) from 159 patients with a positive direct antiglobulin test and monocytes from healthy individuals. The results of the MMA reflected, to a great extent, in vivo haemolysis; 56/63 patients with a positive result, but only 10/96 with a negative one, had evidence of in vivo RBC destruction. Positive MMAs were observed with autoantibodies of all types: warm (42 cases), cold (14 cases), mixed (seven cases), but never in patients with only C3d on their RBCs and no detectable autoantibodies. With warm autoantibodies, the MMA results depended, in part, on the level of IgG sensitization; additional C3d coating acted synergistically to lower the amount of IgG necessary for monocyte interaction to occur. The presence of IgG3 as well as IgG1 on the RBCs was usually associated with a high MMA result; most of these cases, however, also exhibited larger amounts of cell-bound IgG. When the MMA results were analysed in respect of the various types of autoantibodies, differences in the phagocytosis:adherence ratio were observed; in patients with warm autoantibodies, there was a preponderance of phagocytosis, whereas in those with cold and mixed types, adherent RBCs predominated or were found alone.