Fragment X components (Mr 225,000 to 333,000) were distinguished on sodium dodecyl sulfate polyacrylamide gels. Western blotting with monoclonal antibodies to A alpha-chain segments demonstrated that the A alpha-chains of fibrinogen and the largest fragment X components (Mr 285,000-340,000) contained both A alpha 259-276 and A alpha 540-554. Fragment X components of Mr 270,000-285,000 contained A alpha 259-276 but lacked A alpha 540-554, whereas the smallest fragment X components (Mr 225,000-270,000) contained neither A alpha 540-554 nor A alpha 259-276. Studies of the small peptides generated during fragment X formation complemented the studies of the large molecules, by demonstrating peptides containing both A alpha 259-276 and A alpha 540-554 (Mr 41,600-41,800 and Mr 38,700-38,900), peptides containing A alpha 540-554 but not A alpha 259-276 (Mr 20,500-21,000 and Mr 17,300-17,500) and peptides containing only A alpha 259-276 (Mr 23,600-24,000 and Mr 20,500-21,000). Cleavage of B beta 1-42 from the amino terminal ends of the B beta-chains, measured with a specific radioimmunoassay, was linear until 1.6 moles per mole of fibrinogen had been released, and coincided with loss of the central and carboxy terminal A alpha-chain regions, i. e. A alpha 259-276 and A alpha 540-554. Based on present and previously reported data, a model is proposed for the evolution of the heterogeneous group of fragment X derivatives from fibrinogen with the simultaneous release of small peptides.(ABSTRACT TRUNCATED AT 250 WORDS)