A new immunomorphological test for differentiating Leishmania species, based on electron-microscopic revelation of the localization of their specific antigens, is described. The diagnostic value of treating the promastigotes of Leishmania and other Trypanosomatidae with ferritin-labelled antibodies was determined in cross experiments. The immunoferritin test reveals both the serogroup of a given species of Leishmania and specific antigenic differences between different species. However, it fails to differentiate different strains of a given species of Leishmania, which attests to its species-specificity.