A 15‐year‐old Dutch warmblood mare was presented because of lethargy, which had been present for several weeks, and severe anaemia. Total protein was high and serum electrophoresis revealed a monoclonal peak in the alpha‐2 region. Monoclonal immunoglobulin, IgG(T), was detected by immuno‐electrophoresis in serum and urine. Postmortem examination revealed a relatively large number of plasmacytoid cells in the bone marrow and a monotonous population of plasmacytoid cells in the spleen. These findings are suggestive of a plasma cell myeloma.