It sometimes is difficult to diagnose leishmaniasis in tissue sections or in smears, particularly in unusual sites or if few parasites are present in the lesion. Leishmania species must be differentiated morphologically from a variety of other microorganisms, including Toxoplasma gondii, Histoplasma capsulatum, Trypanosoma cruzi, and Penicillium marneffei. We tested the value of monoclonal antibody p19-11 raised against the Leishmania homologue of receptors for activated C-kinase (LACK) as an immunohistochemical marker for amastigotes of Leishmania infantum. We evaluated a total of 117 paraffin-embedded lesions due to L infantum (92 cases), T gondii (15 cases), H capsulatum (5 cases), T cruzi (3 cases), and P marneffei (2 cases). Amastigotes of Leishmania species were detected in 92 (100%) of the leishmaniasis lesions. There were no false-positive LACK immunoreactions in any of the toxoplasmosis, histoplasmosis, trypanosomiasis, or penicilliosis specimens (0/25). We found the anti-LACK antibody p19-11 to be a highly specific and sensitive paraffin-reactive immunohistochemical marker for the confirmation or identification of Leishmania species in tissue sections.