i. selecting a template antibody or antibody fragment from a wild-type antibody or antibody fragment, a mutated antibody or antibody fragment, and a fragment of a wild-type antibody or antibody fragment with a desired property;

ii. evolving a DNA which encodes the template antibody or antibody fragment using one or more evolutionary techniques to create mutant DNAs;

iii. expressing the mutant DNAs to obtain mutant antibodies or antibody fragments at least one of which exhibits a decrease in activity in the assay at the normal physiological pH compared to the same activity in the assay at the aberrant pH; and

iv. screening each mutant antibody or antibody fragment obtained from step (iii) by conducting assays to determine the activity at the normal physiological pH and the activity at the aberrant pH and identifying the one or more conditionally active antibodies or antibody fragments that exhibits a decrease in activity in the assay at the normal physiological pH compared to the same activity in the assay under the aberrant pH,

wherein the assay under the normal physiological pH and the assay under the aberrant pH are performed in assay solutions containing at least one component at substantially the same concentration, said at least one component being selected from hydrogen sulfide at a concentration of 100 μM-100 mM and bicarbonate at a concentration of 25-100 mM.