We have determined the sequence of a cDNA (Tt34c l) encoding a TrYpanosoma cruzi trypomastigote stage-specific 85- kDa surface glycoprotein (gp85). Within the peptide sequence of Tt34cl are two 8-amino acid motifs, Ser-X-Asp-X-Gly-XThr-Trp, that are characteristic of bacterial neuraminidases. Analysis of the Tt34cl sequence predicts the presence of an amino-terminal signal sequence and a hydrophobic carboxy-terminus that is probably replaced by a glycosyl phosphatidylinositol membrane anchor. Gp85 is encoded by an extensive multigene family that is distributed throughout the genome and can be divided into subsets on the basis of oligonucleotide hybridisation patterns. By sequencing products of polymerase chain reaction (PCR) amplification of the 5' end of trypomastigote gp85 mRNA we show that multiple copies of the gene family are transcribed simultaneously in a trypanosome population. Comparison of the sequence of the PCR clones and another gp85 cDNA showed a highly conserved region 5, of the first methionine extending 180 nt into the coding sequence. Insertions and point mutations were observable outside these homologous sequences demonstrating the variant nature of the gp85 mRNAs. (C) 1991 Elsevier Science B.V. All rights reserved.