Purified human thrombin was electrophoresed on SDS-gel. A single major band of approximately 38,000 mol. wt. was obtained. After reduction of disulfide bonds a chain of 33,000 mol. wt. was found, suggesting release of a 5,000 mol. wt. sulfhydryl-attached peptide. The purified human thrombin slowly lysed fibrin in a manner different from that of plasmin and in addition to the release of peptides A and B. This activity may be responsible for some of the apparent heterogeneity of fibrinogen in vivo.