Coombs Control Cells are made up of either a 4% or 0.8% suspension of single donor group O red cells washed to remove all blood group antibodies and then resuspended in a preservative solution. The preservative solution contains neomycin sulphate (0.1 mg/ml) and chloramphenicol (0.34 mg/ml) as preservatives. The cells have been sensitised with IgG.