Circular dichroism spectroscopy was used to investigate the structure of bovine prothrombin fragment 1 (BF1) and related proteins in several environments. The conformational change induced in BF1 by the addition of Mg[II] ions was found to be different from that induced by Ca[II] or Sr[II]. The Ca[II] and Sr[II] conformations appear to differ only slightly from the apo-metal conformation. The conformation of the 1-45 fragment of prothrombin, however, is markedly different than the conformation of the same fragment in the presence of either Ca[II] of Mg[II]; both of the latter structures differ substantially from one another. The presence of phospholipids has almost no effect on the structure of either BF1 or the 1-45 fragment; in the presence of both phospholipids and Ca[II] a structural change is seen for the 1-45 fragment but not BF1 (relative to the protein alone). The addition of phospholipids to the Mg[II]/BF1 structure did not induce a CD-detectable conformational change, while the addition of phospholipids to the Ca[II]/BF1 or Sr[II]/BF1 structures induced a change to a conformation similar in secondary structure composition to the relative apometal structures.