Measurements have been made of the number of available sites on 10 examples of red cells in which the only abnormality appeared to be a quantitative reduction in the expression of D (weak D cells); these estimates were carried out using three monoclonal anti-D antibodies, Fog-1, Brad-3 and Los-2. The values varied with the monoclonal antibody that was used and fell within the range of 170-1,870 sites/cell. A further 3 examples of weak D cells which had brought about immunisation following transfusion were found to have between 390 and 1,470 sites per red cell. The implications of the D site density on the immunogenicity of weak D cells are discussed. The number of sites on red cells with structurally abnormal D (partial D cells) were also estimated, using the antibody Fog-1. Four of the 5 examples of cells of category IVa (probable phenotype Ror) were found to have a high expression of D (range 29,300-41,300), but the available D sites of categories DVa, DVIa, and DVII were considerably reduced (< 500, < 500 and 2,400-7,500 sites/cell, respectively). As a working hypothesis, it is suggested that there are two types of genetic abnormality leading to an abnormal expression of D. First, a defect in genomic DNA leading only to a quantitative reduction in the number of available D sites; this genomic lesion should be termed 'weak D'. Secondly, genomic defects leading to amino acid sequence abnormalities and structural change in the D polypeptide; these lesions should be collectively known as 'partial D'.