The humoral response of twelve strains of mice to immunization with either normal adult human hemoglobin (HbA(,1)) or sickle-cell hemoglobin (HbS) was studied using a quantitative enzyme-linked immunosorbant assay (ELISA). Results indicate the presence of at least two genes that control the magnitude of this response, one of which is located at the 'D-end' of the H-2 complex. H-2 haplotypes s, q and k are associated with a low response, haplotypes d and a with a high response, and the b haplotype with an intermediate response. No difference in the pattern or magnitude of the response was observed when HbS was used as the immunogen in place of HbA(,1). The N-terminal octapeptides of the (beta) chains of HbA(,1) and HbS were synthesized and the presence of antibodies specific for the N-terminal octapeptides was tested. The magnitude of the anti-N-terminal response correlated with that to the intact molecules with one exception. A much lower response to the (beta)('s)-peptide was found in the C57BL/10Sn mice. Monoclonal antibodies (mab) were prepared to HbA(,1) and HbS. Of the 16 monoclonal antibodies isolated, 10 were tested for their ability to bind to 25 Hb variants and the (alpha) and (beta) chains of HbA(,1) and HbS. Eight of ten Mab were directed against (alpha)-chain determinants. One antibody, Mab 7, probably recognizes a determinant located in the amino-terminal portion of the (beta) chain, within the first 26 amino acids. Further analysis of the Mab reactivity suggests a high probability of an additional antigenic determinant near to or including the (alpha)('47) and (alpha)('48) residues. Some mabs were also shown to react against mouse Hb. The implications of these observations will be discussed.