The most common method of blood sample collection for neonatal screening programs for inherited diseases-blood spots on filter paper--is poorly suited for screening of sickle cell diseases by conventional assays because of the denaturing effects of this medium on hemoglobins that affect their electrophoretic identifications. The monoclonal antibody beta (6)-1 specifically recognizes the hemoglobin A beta-chain residue 6 (glutamic acid), that is, the normal counterpart of hemoglobins S and C, and this recognition is unaffected by changes in hemoglobins induced by filter paper storage. The beta (6)-1 immunoassay analysis of 67 prescreened samples extracted from filter paper permitted unambiguous group identification, by virtue of nonreactivity, of the pathologic sickle cell disease phenotypes SS (sickle cell anemia) and SC (sickle cell-hemoglobin C disease), along with the homozygous hemoglobin C phenotype (hemoglobin CC disease). Other phenotypes identified by beta (6)-1 nonreactivity would include S-beta(0) thalassemia, C-beta (0) thalassemia, and beta(0) thalassemia (Cooley's anemia). As systems for collecting newborn blood specimens on filter paper and their transmittal to centralized laboratories are already established in many states, this assay for sickle cell and hemoglobin C diseases could rapidly be combined with other mass screening programs for inborn errors of metabolism.