The Kell blood group protein is a metalloendopeptidase that preferentially cleaves a Trp(21)-Ile(22) bond of big endothelin-3 producing bioactive endothelin-3. Kell is a polymorphic protein, and 25 different phenotypes, because of point mutations resulting in single amino acid substitutions, have been described. It was recently reported that a recombinant form of KEL1 (K, K1) phenotype, expressed in K562 and HEK293 cells, had no endothelin-3-converting activity, in contrast to the common KEL2 (k, K2) phenotype. We demonstrate that KEL1 red blood cells and also a soluble recombinant form of KEL1 protein (s-Kell KEL1) have similar enzymatic activity as the common Kell phenotype. In addition we show that KEL6 red blood cells, which are more prevalent in persons of African heritage than in Caucasians also have endothelin-3-converting enzyme activity and that the recombinant soluble form of KEL6 protein (s-Kell KEL6) has similar K(m) values as the wild-type.