An immunodominant antigen in Trypanosoma congolense-infected cattle is a 69 kDa protein which is conserved among species and developmental stages of African trypanosomes. Immunoscreening of a cDNA expression library identified a 2·35 kbp clone which contains a complete open reading frame encoding a protein of 653 amino acids with a predicted molecular mass of 71 kDa. Protein sequence analyses revealed 45–65% identity with hsp70s from a broad range of organisms, the highest homology being with the mammalian BiP (immunoglobulin heavy chain binding protein). The 69 kDa trypanosome protein shares with other BiP-related molecules two characteristics that are associated with their localization in the endoplasmic reticulum and their function as chaperonins, i.e. a hydrophobic N-terminal signal sequence and a conserved C-terminal tetrapeptide (X)DEL. Divergence between the 69 kDa antigen and other BiP-homologues occurs in the C-terminal region. This may be responsible for the high immunogenicity of the trypanosome protein. The gene for the 69 kDa antigen appears to be present as a cluster of several copies which are not organized in tandem repeats. It is expressed in all developmental stages of T. congolense, but the specific mRNA levels are higher in metacyclics than in other stages.