Patients with sickle cell anaemia (SCA) and individuals who have undergone splenectomy have an increased susceptibility to bacterial infections. Reports of variable capacity of reduction of nitroblue tetrazolium (NBT) by neutrophils from patients with SCA suggest that there may be a polymorphonuclear dysfunction in this disease. In order to assess the role of the spleen in this neutrophil abnormality we carried out the NBT test on neutrophils from normal or splenectomized individuals and from patients with SCA, sickle cell trait and sickle-cell-β-thalassaemia (S-thal). Decreased NBT scores were observed in SCA and in splenectomized control individuals. In the S-thal group, most of the non-splenectomized patients had normal or near normal NBT scores while all splenectomized patients exhibited decreased values. These results indicate a possible contribution of the spleen in the pathogenesis of this neutrophil dysfunction. The mechanism of this participation is unknown.