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In this paper, the lytic activity of two variants of Serratia marcescens against promastigotes of Leishmania braziliensis was studied. In vitro assays showed that S. marcescens variant SM365 lyses L. braziliensis promastigotes, while the variant DB11 did not. Scanning electron microscopy (SEM) revealed that S. marcescens SM365 adheres to all cellular body and flagellum of the parasite. Several filamentous structures were formed and identified as biofilms. After 120min incubation, they connect the protozoan to the developing bacterial clusters. SEM also demonstrated that bacteria, adhered onto L. braziliensis promastigote surface, formed small filamentous structures which apparently penetrates into the parasite membrane. d-mannose protects L. braziliensis against the S. marcescens SM365 lytic effect in a dose dependent manner. SM365 variant pre cultivated at 37 degrees C did not synthesize prodigiosin although the adherence and lysis of L. braziliensis were similar to the effect observed with bacteria cultivated at 28 degrees C, which produce high concentrations of prodigiosin. Thus, we suggest that prodigiosin is not involved in the lysis of promastigotes and that adherence promoted by bacterial mannose-sensitive (MS) fimbriae is a determinant factor in the lysis of L. braziliensis by S. marcescens SM365.