Autoantibodies the specificity of which mimicks Kpb specificity are described in a patient whose phenotype is Kp(a+b-). Manual typing of the patient's red cells revealed a depression of the Kell-related antigens. However, this weak reactivity was not found in the Autoanalyzer using bromeline PVP, the percentage of agglutination being similar to that of red cells of normal blood donors. Alloanti-K antibodies were present in the patient's serum. The specificity of the autoantibodies was considered to be mimicking because they could be absorbed by and eluted from Kpb- and Ko (AET-treated) red cells.