Although the dominance of cyanobacterial species forming water blooms in Sl
ovakia has been well documented for many years, the toxicity of their popul
ations has not been monitored. The results of analysis of hepatotoxic micro
cystins identified and quantified by HPLC with diode array detector are rep
orted, as well as a rapid clean-up method based on centrifugal ultrafiltrat
ion of crude cyanobacterial extract, which can replace conventional time co
nsuming solid-phase extractions. The toxicity of cyanobacterial biomass was
confirmed by bioassay using the crustacean species Thamnocephalus platyuru
s. Level of microcystin-LR, ranged from 0 up to 1445.5 mug/g d.w. Variabili
ty in microcystin contents within different localities was noticed. Not onl
y common bloom-forming species (Microcystis flos-aquae, M. aeruginosa, Plan
ktothrix agardhii or Anabacna circinalis), but also Cylindrospermopsis raci
borskii and Raphidiopsis mediterranca were identified to be dominant specie
s in some cyanobacterial blooms in W Slovakia. Blooms dominated by cyanobac
teria were observed in lakes, fishponds, and water reservoirs. Increased at
tention should be paid to the effects of cyanobacterial toxins on human hea
lth and health of aquatic ecosystems in general.