The comparative mobilities, in citrate agar electrophoresis, of 91 mutant hemoglobins are presented in relation to their molecular structure and in some cases, to their mobilities in other types of electrophoresis. More than a third of theachain mutants (11 of the 27 examined) and half of theβchain mutants (29 of 55) differ to some extent from Hb A. The helical location of the substituted residue is an important determinant of hemoglobin mobility. which is also affected by a complex interplay of other factors. When the data are combined with those of several other types of electrophoresis, they often provide presumptive (or in some cases highly specific) identifications of mutant hemoglobins and hemoglobinopathies.