Detrimental and beneficial properties of prokaryoticand eukaryotic microalgae may be qualifications givingthem posibillities for their biotechnologicalutilization. Phycotoxins and related products mayserve as material for useful drugs. Cyanotoxins arean example. Bioassay screening methods are used todetect specific biological activity (e.g. antibioticeffects). A study demonstrating antibacterialproperties among cyanophytes showed that these effectswere caused by substances produced by both toxigenicand non-toxigenic strains. Differences inbiosynthetic activity are influenced by the type ofclones and by the culture conditions (genetic andbiochemical variation). Experience of recentscreening (1997–1998) of compounds from cyanophytes –including 25 species from 14 genera – display thediscovery of more than 30 different antibioticsubstances. The further isolation of newpharmaceuticals from natural resources is becomingdifficult, if not uncommon species of producerorganisms are taken into consideration.