NEW MATERIAL: A fibrin/fibrinogen derivative in which all or partial methionine groups in a basic amino acid sequence are replaced with methionine sulfoxide groups.

USE: An affinity reagent.

PROCESS: Fibrin, a fibrin or fibrinogen decomposition product, or a fibrin or fibrinogen partial sequence is treated with an oxidizing agent such as a halogen amine (example: chloramine, etc.), in conditions to convert all or partial methionine groups contained in the amino acid sequence of those substances into methionine sulfoxide groups. The reaction is preferably done at pH 7-9 in the presence of a chelate forming agent and a surfactant.