Mouse macrophages, grown in continuous cell culture at 37°C, were inoculated with the promastigote stage of various human pathogenic Leishmania species. Under these culture conditions, the parasites rapidly entered the cells and transformed into amastigotes. Two or 3 days after inoculation, the infected macrophages were mixed with washed human erythrocytes and were fed to female sand flies (Phlebotomus papatasi and Lutzomyia longipalpis) through a chick skin membrane. Within 7–10 days after feeding, large numbers of promastigotes were observed in the anterior portion of the insects' guts, indicating that the infected sand flies were capable of transmitting the parasites by bite. This relatively simple and rapid technique should facilitate studies on the biology of Leishmania in their insect vectors. It also eliminates the need for animals as a source of amastigotes.