Marine dinoflagellates of the genus Alexandrium are known to cause lytic effects on a wide range of photosynthetic and heterotrophic protists. These effects include immobilization, total cell lysis and/or cyst formation in the target species. High cell concentrations of Alexandrium are also known to cause gill damage associated with mass fish mortalities. There is ample evidence that these deleterious effects are not due to known phycotoxins, such as PSP toxins and spirolides, produced among various Alexandrium strains. Chemical structures of the lytic compound(s) are unknown and there is little information on their basic physical-chemical properties and mode of action. Here we present preliminary results towards chemical characterization of lytic compound(s) excreted by Alexandrium tamarense. Stability under various physical and chemical conditions was tested to further define the nature of the compound(s). Bioactivity was stable at <20 ºC for several months in sterile seawater, within a pH range from 6 to 10, but it was decreased with increased air and vessel surface. Chromatographic isolation of the bioactive fraction was achieved after application of solid-phase extraction. Size-fractionation by ultrafiltration and gel permeation chromatography indicated that the bioactive compound(s), or at least the secondary structures, comprise large complexes (estimated >500 kD).