Antibodies to various high-frequency antigens were used to test red cells of members of 12 families of Lu(a-b-) propositi carrying the In(Lu) gene. Kna, Kna-like, McCa, and Sia usually are expressed more weakly on cells of the Lu(a-b-) members than on those of their relatives who were not Lu(a-b-). In most families, In(Lu) also suppresses Yka and Csa expression. Some antiserums are more efficient than others of the same specificity in demonstrating the weakened expression. The expression of Ch and Rg antigens on red cells is not affected by In(Lu).