Preferential processing of big ET-3 by RBCs of common Kell phenotype: comparison with Ko(null) phenotype. Common Kell phenotype and Ko(null) RBCs were incubated for 15 minutes at 37°C with big ET-1, big ET-2, or big ET-3, and the endothelin generated was measured by EIA. A 100% activity is that of wild-type RBCs with big ET-3 as substrate. (▨) RBCs of common Kell phenotype (wild-type); (▪) Ko(null) RBCs.