SUMARY Melaleuca leucadendron L is a tree commonly known as cayeput, being an invasive, undesirable species in Cienaga de Zapata, in the province of Matanzas. According to the referred usages, it might be exploited in a reasonable way as an antimicrobial agent. As our objective we pretended to determine antimicrobial activity in the fluid extract of the leaves of this tree wildly growing in the before mentioned territory. The extract was obtained by repercolation and the assay was carried out in vitro, using the methods of diffusion in agar and dilution in a liquid media. Different concentrations of the extract were used against bacteria, yeast, and filamentous fungi. The microbicidal effect of the extract was demonstrated for all the tested microorganisms, establishing values of minimal inhibitory, minimal bactericidal and minimal fungicidal effects of the concentration. The effect kept steady during four years. These results confirm the purpose of using as antimicrobial the raw vegetable material existing in excess in the region. It is recommended to take into account the matters discussed in the study according to the undesirable effects, so other studies are required to deepen in that sense. The evaluation risk-benefit should be observed to use the tree in the medical practice with the precautions also demanded by other well-known antimicrobials.