An antiserum identifying a red cell determinant expressed by Rh:33 and by some "new" depressed Rh phenotypes.

An antibody detecting a determinant on Rh:33 cells and cells with depressed C and/or e antigens was separated by the absorption and elution technique from a serum with antibodies to several low-frequency antigens. The determinant, tentatively named FPTT, has a frequency of about 0.0094 percent in the South of France.