The antibody-coated human erythrocyte and the antibody-coated ox erythrocyte rosette assays (EAHu and EAOx) were compared to detect Fc gamma receptors on human peripheral blood lymphocytes. Two incubation conditions were examined: 1 h at room temperature and overnight at 4 degrees C. In healthy persons, in patients with Hodgkin's disease and in patients with non-Hodgkin lymphoma (NHL) the mean percentage of EAHu-rosette-forming cells (EAHu-RFC) increased significantly when the incubation was carried out overnight at 4 degrees C instead of 1 h at room temperature. This increase was caused by Fc gamma receptor-bearing T cells. In the case of EAOx-RFC only a slight increase was found. The percentage of EAHu-RFC and EAOx-RFC differed significantly in the healthy group after the overnight incubation and in the NHL group after the 1-hour incubation. When comparing the mean percentage of EA-RFC in the patient groups with that of the healthy persons significant increases were observed: EAHu-RFC in patients with Hodgkin's disease in the overnight incubation and EAOx-RFC in patients with Hodgkin's disease and NHL in both incubation conditions. In patients with chronic lymphocytic leukemia (CLL, B-cell type) the mean percentage of EAHu-RFC was very low, however that of EAOx-RFC was moderate to high. It is concluded that in the two rosette assays the antigen-antibody complexes may have different avidities to different lymphocyte subpopulations, and that incubation conditions may have an influence on this avidity.