An antibody is described which defines a new high frequency red cell antigen, Duclos, whose expression seems to require the presence of both U and Rh fundamental antigens. Apart from the antibody maker's own red cells the only nonreactive samples were from Rhnull U impaired individuals, one example of which was shown however to yield very slight amounts of antibody through absorption-elution tests. Rhmod U weak cells gave very depressed and Rhnull U positive or Rh common U negative cells moderately depressed reactions. The proposita's red cells had an apparently normal Rh-LW condition but their U antigen was significantly decreased. No further Duclos negative individual was found by screening 8,500 blood donors in the Paris area.