Serological studies on blood of selected individuals show that red blood cells from individuals genetically JkaJk and JkbJk give single dose agglutination reactions with anti-Jka and anti-Jkb respectively, but react as strongly with anti-JkaJkb as do cells from random individuals. A Jk(a+b-) Oriental person has been found whose red blood cells give a single dose reaction with anti-Jka, but react more weakly than do cells from random people in tests with anti-JkaJkb. The results support a conclusion that the antigen recognized by anti-JkaJkb is a distinct and separate antigen of the Kidd blood group system.