The effect of serum obtained from rats with acute and chronic renal failure on phagocytosis was evaluated in macrophages cultured "in vitro". The erythrophagocytosis of these cells was lower after incubation with serum from rats with acute and chronic renal failure when compared to normal plasma. This inhibitory process persisted when the serum was previously filtered through a Millipore filter PM-30, but it was abolished when the serum was filtered through a Millipore filter PM-10. These data suggest that uremic plasma displays a inhibitory factor of phagocytic cells of a molecular weight between 10.000 and 30.000 daltons.