The nature of the cross-reacting groups on two variant surface glycoproteins of Trypanosoma brucei has been investigated after isolation of glycopeptides produced by extensive proteolytic digestion of the proteins. One variant yielded two glycopeptides after pronase digestion, one of which was the glycosylated C-terminal aspartic acid. In a second variant there are two carbohydrate groups close to the C-terminus. Considerable heterogeneity in the size of the sugar attached to an asparagine residue was detected whereas the C-terminal serine was glycosylated but showed no size heterogeneity. For both variants it was shown that the C-terminal glycosylated amino acid (either aspartic acid or serine) was responsible for the immunological cross-reaction between distinct variant glycoproteins. The stability of the cross-reacting determinant was also investigated.