Sections from skin lesions and draining lymph nodes of patients with post kala-azar dermal leishmaniasis were examined using an immunoperoxidase method and a monoclonal antibody directed against Leishmania donovani. Parasites were detected in 22 of 25 biopsies (88%). In parallel sections stained by haematoxylin and eosin, parasites were detected in only 5 of 25 of the biopsies (20%). The demonstration of whole parasites or parasite antigen is diagnostic.