1. The amino acid compositions of human fibrinogen and three intermediate anticoagulant derivatives were determined by column chromatography. The derivatives were isolated by ammonium sulphate fractionation and column electrophoresis from solutions of fibrinogen undergoing spontaneous breakdown. One derivative, isolated as the large electrophoretic peak at the end of the clottable period (100% CP) of the parent fibrinogen solution, was labelled LP100 and others obtained at twice this period (200% CP) were designated as LP200 and SP200 (LP, large peak; SP, small peak). 2. Maximal `molecular' weights of approx. 294000 for LP100, 137000 for LP200 and 37000 for SP200 were calculated for the protein moieties. At least 265 amino acid residues must have been lost from each fibrinogen molecule during the formation of LP100, and 1362 during the formation of the other two derivatives. 3. Only one derivative (LP200) had a partial specific volume ([unk] 0·725ml./g.) different from that of fibrinogen ([unk] 0·721ml./g.). 4. No significant differences in refractive index at 589mμ were detected. 5. Calculation of the total number of ionizable groups/105g. of each protein moiety showed a preponderance of the following numbers of negative charges: 22 in fibrinogen; 24 in LP100; 26 in LP200; 49 in SP200. The isoionic points were estimated to be approx.+0·03pH unit (for fibrinogen), −0·06pH unit for (LP100) and +0·28pH unit (for LP200) from the pK of imidazole, and 0·78pH unit above the average pK of aspartyl and glutamyl ions (for SP200). These figures agree closely with experimentally determined values of the isoelectric point of fibrinogen and its derivatives.