Arenosclerins A-C and haliclonacyclamine E, new tetracyclic alkylpiperidine alkaloids isolated from the marine sponge Arenosclera brasiliensis, were subjected to antimicrobial and cytotoxic bioassays. Fourteen samples of microorganisms were used: Candida albicans, Staphylococcus aureus, Escherichia coli, and 12 antibiotic-resistant bacteria isolated from hospital environment. The minimum inhibitory concentration activity of each alkaloid was determined. The four compounds displayed antibacterial activity, but no antifungal activity against C. albicans. Haliclonacyclamine E and arenosclerins A and C were active against a larger number of bacteria strains than arenosclerin B. However, arenosclerins B and C presented more potent antibacterial activity. The alkaloids displayed inhibitory activity against both Gram positive and Gram negative bacteria. Cytotoxicity bioassays using the MTT method showed that these compounds present cytotoxic activity against human HL-60 (leukemia), L929 (fibrosarcoma), B16 (melanoma) and U138 (colon) cancer cell lines at concentrations between 1.5 and 7.0microg/ml. The results obtained indicated that A. brasiliensis alkaloids have a potent toxic activity. The broad cytotoxic and antimicrobial activities presented by A. brasiliensis alkaloids suggest a defensive role of arenosclerins and haliclonacyclamine E against microbial infection and/or the action of potential predators at the sponge's natural habitat.