168 Rh specific monoclonal antibodies (Mabs) were evaluated in 43 laboratories. Red cells from 63 Rh variant phenotype donations were circulated in panels to evaluators. Circulation was organised to ensure adequate duplicate testing. Evaluators were asked to test each Mab at 3 dilutions in two specified techniques against normal phenotype cells, panel cells received and any other Rh variant cells they had available. Results were analysed by expressing the sum of reaction grades for each Mab with each variant cell as a percentage of the sum of reaction grades of that Mab with normal phenotype cells. Reactions with enzyme-treated cells were found to be highly variable between laboratories, and further analysis of these results was not attempted. Anti-D Mabs were sorted into groups which had the same pattern of reactions with different phenotype cells. 24 such patterns were identified, 17 corresponding to previously reported patterns and 7 new ones.