B-lymphoblastoid cell lines transformed by Epstein-Barr virus were produced from cells obtained from a hyperimmunised donor with serum anti-D activity against category D^VI red cells and enriched for this activity by rosetting with category D^VI red cells. Three clones produced IgG1 anti-D and had stable cell growth and continuous secretion of antibody in prolonged culture. The monoclonal antibodies reacted with category D^VI red cells, when assessed manually and in an automated blood grouping system, and are useful blood grouping reagents for the detection of the category D^VI phenotype. Using a radiometric technique, the number of antibody molecules bound to category D^VI red cells from 5 individuals was estimated to range from 2,800 to 11,200 per cell. Five percent of blood donors classed as D^u in the south western region were found to have the category D^VI phenotype.