A comparison is made of the four main components of an homozygous variant (A or D,D,) of bovine serum transferrin. These are designated I-IV in order of increasing mobility in electrophoresis at pH 7.5. Components I, 11, I11 and IV have 2,2,3 and 3 residues of sialic acid per transfemn molecule .and appear to correspond to components 2a, 2b, 3a and 3b respectively of Strati1 & Spooner (1971). The difference between components I and I1 and between 111 and IV does not reside in sialic acid differences. On the basis of peptide maps of reduced carboxamidomethylated components, urea-starch gel electrophoresis and quantitative se-.quence studies, it is concluded that components I1 and IV have a scission in the peptide chain. By homology with the sequence of MacGillivray et al. (1977) for human serum transferrin it is suggested that the scission occurs between residues 55 and 54 from the C-terminus and this portion of the chain has a 'molecular' weight of ca. 6000. The implications 'are briefly discussed.