A 'new', rare blood group antigen, Ola (Oldeide), is described. Only one Ol(a+) person was found among the 7,151 blood donors tested. Examination of the family of the proposita revealed 10 additional Ol(a+) persons. The study indicated that Ola is inherited as a Mendelian dominant character and segregates independently of the ABO, MNSs, P, Rh and Kidd blood group systems, of the ABH secretor genes, and of sex. Ol(a+) family members had depressed Rh antigens, while the Ol(a-) members had Rh antigens of normal strength. The depression was particularly pronounced for the C and E antigens, less marked for the D antigen. Individual antisera differed in their ability to demonstrate the presence of the antigens. Depression of the c and e antigens could not be demonstrated conclusively with the antisera available.