A proportion of Melanesians possess an outstandingly weak I^T antigen. Their red cells are I^T negative in the sense that cord bloods are I negative. Of Papuan coastal dwellers, 15.4% of 532 subjects tested were I^T weak. This proportion is not constant among Melanesian ethnic groups. Of 419 New Guinea Highlanders, none was I^T weak. Similarly, of 171 Caucasians tested, all had normal IT content. The phenomenon can be demonstrated on Melanesian cord bloods, and family studies confirm the genetic basis, ITw presumably reflecting a recessive homozygous state. The place of I^T in the li system remains obscure, but the demonstration of genetic variation in this antigen independent of I and i strength, and occurring frequently in certain ethnic groups, clearly reveals hitherto unsuspected complexity.