From the diversity of size heterogeneity among equine transferrin (Tf) types and quantitative variants between T⁺ and T^- components composed of Tf types, the Tf types could be classified into three groups as Tf-D·F·H, Tf-O·R and Tf-X subgroups. To certify furthermore the results, the equine Tf types were subgrouped from peptide cleavage patterns produced by peptide mapping using four proteases (α-chymotrysin, V8 protease, papain and endoprotease Asp-N). The results obtained are as follows: 1) When Tf·D, F and H types were treated by the four proteases, these three Tf types had almost the same peptide cleavage patterns. Although there were respectively some different band patterns between each peptide cleaved in Tf·O and R types, they had more mutually similar points than those of the Tf-D·F·H subgroup. This means that Tf·O and R types may be mutually specialized, but it was guessed that they can be basically classified into the same subgroup in view of having many of the same peptide cleavage components. The four peptide patterns which were cleaved donkey's Tf components, were each very similar with those of equine Tf·O and R types. 2) T⁺ and T^- components of Tf·D and R types were each cleaved by V8 protease and α-chymotrypsin, and both components of the Tf·D type showed almost same peptide pattern, and there were distinct differences when the two components of the Tf·R type were digested by the latter protease. From analysis by peptide mapping, Tf·O and R types might be newly subgrouped, but it was again recognized that the peptide structures of the Tf·O and R types were each distinctly different from those of the Tf-D·F·H subgroup. Accordingly, the equine Tf types could be classified into two groups as Tf-D·F·H and Tf-O·R types by peptide mapping. Then the T⁺ and T^- components in the Tf·R type had basically different peptide structure, respectively.