Microorganisms have been showing augmented resistance towards antimicrobials, being microbial resistance, nowadays, one of the biggest problems of public health. Thus, there is an increasing interest in the development of new strategies of microbial control, namely the ones based on natural products, especially from plants, such as lectins. So, it is of utmost importance to test new antimicrobial compounds, especially with a wide range, against bacteria and fungi. Therefore, the main aim of this thesis was to evaluate the effect of new lectins against both Gram-negative and Gram-positive bacteria, as well, as yeast. Moreover, these lectins were assessed against planktonic and sessile cells. Four Diocleinae lectins: Canavalia ensiformis, Canavalia brasiliensis, Canavalia maritima and Canavalia boliviana, were used in this study. Their effect was assessed against two Gram-positive bacteria (Staphylococcus epidermidis, Staphylococcus aureus), two Gramnegative bacteria (Pseudomonas aeruginosa, Klebsiella oxytoca) and two yeasts (Candida albicans and Candida tropicalis). The effect was evaluated on microbial planktonic growth, early-stage adhesion, on biofilm biomass and biofilm viable cells. The four lectins showed different activities (inhibitory or stimulatory effect) on planktonic growth, early-stage adhesion and biofilm formation, for the same microorganism. In general, the inhibitory effect of lectins was most notable on planktonic growth than on biofilm formation. Although there are few differences in the inhibitory capacity of lectins, K. oxytoca can be considered the microorganism that suffered lower inhibition. Interestingly, the results demonstrated that activities of lectins tested were species-dependent, namely, their action was different between the two Gram-negative species, as well as the two Gram-positive and the two yeasts. This highlights that the interaction between the lectin and the cell is of high specificity. So, in conclusion, due to the specificity of the lectins assayed, it could be of major interest to improve their potential (synergic effect) by using more than one at the same time or combined with conventional antimicrobial agents.