biological activity of an aqueons extract of the sponge aplysina lacunosa (porifera: aplysinidae). the aqueous extract and protein precipitate of aplysina lacunosa (pallas, 1776) were studied to assess their hemagglutinating, hemolysing, antibacterial, and antifungal activities. specimens of the marine sponge were collected in el morro de tigüitigüe, santa fe, sucre state, venezuela. the active protein was separated by molecular exclusion chromatography and its molar mass estimated by sds-page electrophoresis. the sponge a. lacunosa has a protein with a molar mass of about 43 000 daltons which is capable of agglutinating human erythrocytes of the blood groups a, b, and o in a strong and unspecific mode. the assayed samples did not evidence any hemolysing activity. as for the antibacterial assay, only the aqueous extract was able to inhibit the growth of enterococcus faecalis, bacillus cereus, escherichia coli, and salmonella enteritidis, with inhibition halos of 24, 20, 24, and 22 mm, respectively. none of the samples exhibited antifungal activity. the chemical analysis of the aqueous extract revealed the presence of several secondary metabolites. it is presumed that its hemagglutinating activity is mediated by agglutinative proteins. the antibacterial activity could be attributed to the presence of saponins, alkaloids, tannins, and polyphenols, which are highly antimicrobial compounds. poriferans are a rich source of bioactive compounds that can be used in the development of new drugs potentially useful in medicine. rev. biol. trop. 54 (suppl. 3): 189-200. epub 2007 jan. 15.