By using Lowicryl K4M, a kind of low temperature embedding resin, to embed promastigotes of Leishmania donovani at -30 degrees C, the protozoal structure was well preserved when visualized by electron microscope. We have determined the distribution of the promastigote antigen of human and canine isolates of Leishmania donovani from Sichuan using two monoclonal antibodies combined with biotinylated goat antimouse IgG and streptavidin/colloidal gold conjugates. The results showed that the antigen recognized by the protective monoclonal antibody 2H6-E3 was mainly distributed in the outer side of promastigote membrane of Leishmania donovani Sichuan canine isolate. According to the high density of localized gold particles this antigen might be abundant in the promastigote membrane. This result facilitates further research on the component, property and function of this antigen. Monoclonal antibody 1H7-C2-E7 recognized the promastigote antigen of both human and canine isolates of Leishmania donovani from Sichuan, suggesting that these two isolates might share a common antigen, and 1H7-C2-E7 antibody might be used to diagnose the leishmaniasis caused by these two isolates.