A sixty-year old female was referred to the Internal Medicine Department for the treatment of a diffuse high-grade non Hodgkin's lymphoma. She presented episodes of fever in context of neutropenia (neutrophils 0.35 x 10(9)/1 from 1.6 x 10(9)/1 white blood cells). Hemoglobin level was 8.2 g/dl and platelets 132 x 10(12)/1. A monoclonal IgM-Kappa protein (48 g/l) was detected in her serum. A direct antiglobulin test on the red cells proved positive with anti-C3d but not with anti-IgG antiglobulin, due to the presence of an IgM cold antibody with a serological anti-i specificity. The IgM antibody was found on the patient's neutrophils as well as in her serum. The antibody recognized all neutrophils tested in conventional serological tests whether the neutrophil phenotypes in systems NA, NB, and 5. It was demonstrated that it recognized the i antigen expressed on the neutrophils. These results suggest that a cold agglutinin anti-i might be responsible for neutropenia in some patients.