Several highly antigenic Xgtl 1 cDNA recombinant phage were isolated from a Trypanosoma cruzi bloodstream trypomastigote cDNA library by immunoscreening (1). One of these, L-6, encoded the C-terminal portion of a T. cruzi acidic ribosomal P protein. Using an oligonucleotide derived from a specific internal region of the L-6 cDNA, and another one corresponding to the first 20 nucleotides of the T.cruzi splice leader sequence, SL (2), we amplified, by RNA-PCR, the 5' end of this mRNA. The amplified cDNA, together with the L-6 cDNA, span the complete mRNA sequence of an acidic ribosomal P1 tpe protein, TcP1. It is 541 nucleotides long, and includes an ORF of 330 nucleotides encoding a 109 amino acid polypeptide, with an estimated pI of 5.13. It shares 51% and 40% homology with the corresponding human P1 (3) and Saccharomyces cerevisiae Plea proteins (4), respectively (Figure 1), and it shows typical Nterminal features of the ribosomal P1 proteins such as the serine residue in alignment position 3, and the amino acid sequence LAC, alignment positions 8-10 (Figure 1, and reference 5). Moreover, the TcPl sequence presents an internal amino acid motif GG(V/P)APAAG, alignment positions 67-74, only found in P1 proteins of higher eukaryotic organisms (5). It is noteworthy that TcPl also shares a great degree of homology with the two other small molecular weight T.cruzi ribosomal P2 type proteins, TcPJL5, 45%, and TcP2b, 47%, (6). All three T.cruzi proteins conserve: a) the characteristic P1 amino acid motif KAXGXXV beginning in alignment position 32 (Figure 1), b) a large number of alanines residues clustered in the alanine-rich hinge region, and c) the serine free C-terminal sequence EEEEDDD-MGFGLFD (Fig. 1).