The regulation of syngeneic erythrophagocytosis (EP) by macrophages (M phi) harvested from young and old NZB mice was examined by spectrophotometric assay and morphological observation. Peritoneal exudate M phi from young NZB mice weakly ingested syngeneic red blood cells (RBC). T cells derived from old NZB mice accelerated ingestion of RBC by young M phi. On the contrary, T cells from young NZB mice suppressed EP by young T cells appeared clearly when they were added to M phi derived from old mice, which ingested syngeneic RBC actively without help by old NZB T cells. Namely, such an active EP by old M phi was completely suppressed when they were incubated with young T cells. Simultaneous addition of both young and old T cells to either young or old NZB M phi with RBC suppressed the EP. Pretreatment of young T cells with anti-Lyt 1.2 antibody and complement (C) made the suppressive activity prominent, and preincubation with anti-Lyt 2.2 and C eliminated the suppressive activity, but gave rise to the enhancing activity. Young T-cell homogenates added to younger or old M phi together with RBC did not reveal suppressive activity for EP, and on the contrary facilitating activity appeared predominantly. Young and old T-cell homogenates added together to young M phi did not suppress EP. The largest of T-cell-factor accelerating EP was M phi, but not RBC. M phi with active EP belong to Ia-bearing subpopulations.