Classification of subjects with a partial D antigen is traditionally performed with immune anti-D sera. The development of monoclonal antibodies enables a fine analysis to be made of the specificity of the epitopes that are present or missing in these cases. A systematic search in a Caucasian donor population of 17,500 revealed 8 unrelated male individuals (frequency 0.05%) with a red cell phenotype characteristic of partial D category VI, but without anti-D in their serum. The relation to the 'classic' partial D category VI was investigated and is discussed, as is the observed serological heterogeneity of the partial D category VI group. Clinical consequences for the prevention of immunization of these subjects are mentioned.