The interaction between dimethyl sulphoxide-induced Friend leukaemia erythroblasts (IFLE) and cultured syngeneic mouse peritoneal macrophages (PM phi) was assessed radiochemically as well as by light and electron microscopy. The data revealed that a small number of the IFLE either became attached to or were ingested by the PM phi and that the interacting IFLE were usually well differentiated. Interactions of a similar nature were also seen when IFLE were co-cultured with macrophages derived from mouse bone marrow. The extent of interaction between IFLE and PM phi provides a useful system in which mechanisms underlying ineffective erythropoiesis can be defined.