Cattle transferrin (Tf) was purified from serum of variant A and four bands were isolated. The peptide patterns of these bands when cleaved by proteases and by cyanogen bromide (BrCN) were compared, using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Variant A displays two groups of molecules--large (L) and small (S)--on SDS-PAGE; the molecular weight of the L bands is 78,400 +/- 1700 and that of the S bands is 72,000 +/- 1700. However, S-band molecules could not be produced artificially by heat treatment of L bands in the presence of SDS and 2-mercaptoethanol. Since deglycosylated Tf also showed molecular weight heterogeneity, the sugar moieties of Tf other than sialic acids were not the cause of the heterogeneity. These results suggest that heterogeneity within a given variant is due to the presence of two kinds of molecule of different molecular weight. The peptide patterns of L and S bands produced by proteases and those produced by BrCN were distinctly different from each other. However, the stepwise degradation patterns of L and S bands resembled each other when treated with both chymotrypsin and BrCN. This suggests that L-band molecules differ from S-band molecules only in the presence of an additional carboxyl-terminal peptide.