Correct donor D typing is critical to prevent recipient alloimmunization. No method can detect all variants, and the immunogenicity of many variants is unknown. Routine ABO and D serologic typings are performed in our laboratory by automated microplate testing. Until 2011, routine confirmation of D- status of first-time donors was performed by the manual tube indirect antiglobulin test (IAT); this was replaced by automated solid-phase testing including weak D testing by IAT. Selected donors are investigated by other methods. We describe four weak D type 67 (RHD*01W.67) donors whose samples tested as D- by automated microplate and manual methods but were later determined to be D+ by automated solid-phase and RHD gene analysis. Solid-phase serologic and molecular typing results of all four donors were identical. It was identified that the donors are of English-Irish descent; two are brothers and the others are cousins. Transfusion of blood from one of these donors likely resulted in alloimmunization to D in one of three recipients tested since no other documented exposures were identified. Lookback studies determined that two other D- recipients were not alloimmunized.