Fc-receptor on ZK1 cells mediated phagocytosis of opsonized sheep red blood cells (SRBC). ZK1 cells were plated at 1 × 106 cells/well in a 6-well plate containing a sterile micro cover glass per well in RPMI complete medium for overnight at 37°C. Unopsonized (as negative control) or preopsonized SRBC were plated on monolayer of ZK1 cells at a ratio of 20:1 and incubated at 37°C for 1 h. After removal of free SRBC by medium exchange and lysis by osmotic shock, the cells on the cover glass were fixed and stained with a modified Wright stain, subsequently examined by light microscopy. Panel A, ZK1 cells were unable to ingest unopsonized SRBC after lysis. Some free SRBC were present without lysis as background. Panel B, approximately 80% of ZK1 cells were positive phagocytosis of opsonized SRBC.