<jats:title>ABSTRACT</jats:title><jats:p>Several members of the<jats:italic>Trypanosomatidae</jats:italic>family, when freshly isolated from their mammalian hosts, have immunoglobulins adsorbed to their cell surfaces. However, a significant portion of these antibody molecules is not parasite specific, i.e., the immunoglobulins are bound to the parasite's cell surface molecules via noncognitive interactions. It has been proposed that this noncognitive adsorption of immunoglobulins to the parasite is mediated by an Fc-like receptor present in several members of the<jats:italic>Trypanosomatidae</jats:italic>family. However, the molecular identification of this receptor has never been defined. Here, we describe the cloning of a gene encoding a protein that might represent this molecule. The gene, named<jats:italic>Lmsp1</jats:italic>, was cloned by screening a<jats:italic>Leishmania major</jats:italic>cDNA expression library using a rabbit antiserum.<jats:italic>Lmsp1</jats:italic>is present in both<jats:italic>Leishmania</jats:italic>and<jats:italic>Trypanosoma</jats:italic>and is expressed in all developmental stages of these parasites. The predicted protein has a molecular mass of 16.6 kDa and contains an RGD sequence starting at residue 104 and three cysteine residues at positions 55, 74, and 116. The purified recombinant protein strongly binds to normal immunoglobulins of various animal species (humans, rabbits, sheep, goats, guinea pigs, donkeys, rats, and mice) and the binding to human immunoglobulins appears to be immunoglobulin G (IgG) and IgM isotype specific. Moreover,<jats:italic>Lmsp1</jats:italic>binds to both purified Fc and Fab fragments of IgG from both humans and rabbits. The mapping of the<jats:italic>Lmsp1</jats:italic>epitopes that bind human IgG revealed that different sequences of the molecule bind to Fc or Fab. In addition, fluorescence-activated cell sorter analyses with a specific rabbit anti-<jats:italic>Lmsp1</jats:italic>antiserum showed that<jats:italic>Lmsp1</jats:italic>is associated with the parasite's cell surface. Finally, inhibition experiments point to an active role of this molecule in the immunoglobulin-mediated attachment and penetration of<jats:italic>Trypanosoma cruzi</jats:italic>in its macrophage host cells, thus suggesting that<jats:italic>Lmsp1</jats:italic>is a putative<jats:italic>Trypanosomatidae</jats:italic>immunoglobulin receptor.</jats:p>