A purification method to obtain a fibrinogen preparation with a high proportion of intact molecules and free from contaminating plasminogen and Factor XIII was developed. Such fibrinogen preparations were used in cross-linking and plasmin-digestion studies to investigate the possible involvement of the carboxyl terminal regions of the A-chain in a high affinity calcium-binding site. The results of these investigations gave no evidence to support such a role for the carboxyl terminals of the A-chains. Highly intact fibrinogen preparations were also used in experiments to re-assess the number of high affinity calcium-binding sites in the molecule. The technique of flow dialysis was employed. Results from these experiments were in agreement with previously published data that there are three high affinity calcium-binding sites in fibrinogen. Indirect evidence was obtained which suggests that the chelator EGTA binds to the fibrinogen molecule.