Complement receptor type one (CR1) is a polymorphic glycoprotein, present on red blood cells (RBCs), that carries the Knops blood group system antigens. Since Knops system antigens can vary in strength, we investigated whether CR1 deteriorated upon storage, thus affecting Knops blood group system antigen reactivity. Units of whole blood were collected in CPDA-1 and evaluated at day 0 and day 35 for antigen strength, using routine serologic techniques. CR1 was quantitated by an enzyme-linked immunoassay and by flow cytometry measurements. The CR1 expression polymorphism was ascertained by a