The interaction of promastigotes of Leishmania donovani with mouse peritoneal exudate cells (cep) under the influence of Chymotripsine was studied. The promastigote adhered to the cep terminally and marginally. At 10-30 minutes postchallange, adhesion was absent in the treated group, and only after two hours was there any adhesión (24% in comparison with the controls). The experimental group was markedly deficient in all the parameters of activity measured: attachment, penetration, intracellular multiplication and division of flagellates forms in comparison to controls. The Chymotripsine favored the development of intermediate flagellates forms, these being spheroid or pyriform, with example of an aberrant form having the anterior end cylindrical similar to a choanoflagellate. It is suspected that the Chymotripsine effectively reduces protein or peptide fragments; these may have been so small as to conceal other ligands associated with macrophage-parasite adhesion.