The gene B protein (GBP) is one of the products of the LmcDNA16 gene family, a cluster of related but non-identical genes that are differentially-expressed during the Leishmania life cycle. This protein, which is found on the surface of infective stage parasites, contains an extensive region of proline-rich amino acid repeats, constituting 45% of the total protein. The structure and stability of these repeats have been investigated in a number of L. major strains by polymerase chain reaction (PCR) amplification and Southern blotting. Data reported in this paper demonstrate variability between strains with respect to the number of repeats encoded by GBP, although those strains isolated within adjacent geographical regions have conserved repeat structures. The data also reveal that some parasite lines have additional repeat sequences within a second, related gene in the LmcDNA16 array. Western blotting experiments have established that these sequences are expressed in vivo, indicating that L. major strains are heterogeneous in their surface complement of gene B repeat-containing proteins.