Ten thiadiazine derivatives were tested in vitro for antiparasitic effects against both extracellular promastigotes and intracellular amastigotes of Leishmania amazonensis. The results showed that the evaluated compounds exhibited a strong antiproliferative activity on all developmental stages of the parasite. The minimal inhibitory concentration and the 50 % effective concentration values against the promastigote were 2.1-5.1 microg/ml and 0.6-1.8 microg/ml, respectively. The tested compounds caused an irreversible inhibition of the promastigote growth either after 1 h of treatment with 10 microg/ml or after 24 h with 1 microg/ml. Also, the thiadiazine derivatives were active against amastigotes producing between 12 and 89 % of reduction of infection at 100 ng/ml. However, the compounds exhibited high toxicity and provoked inhibition of the phagocytosis in the murine host cell.