A 9-year-old spayed female English Mastiff was referred for outpatient ultrasound due to a 3-week history of weight loss, vomiting, and decreased appetite. Abdominal ultrasound showed multiple splenic masses of varying sizes and serum chemistry panel showed hyperglobulinemia. Cytologic examination of fine-needle aspirates of the splenic masses indicated a mast cell and plasma cell collision tumor. Results of serum and urine protein electrophoresis and immunofixation indicated the plasma cell neoplasia was producing IgA immunoglobulins.