In the present study, we have evaluated the effectiveness of a passive sampler for polar organic chemicals to accumulate a group of widespread and hazardous tumor-promoting toxins produced in cyanobacterial water blooms-microcystins (MC). The previously optimized configuration of the sampler based on polycarbonate membrane and Oasis HLB sorbent (2.75 mg/cm(2)) was validated under various exposure scenarios in laboratory and field. Calibration of the passive sampler conducted under variable conditions and concentrations of MC revealed linearity of the sampling up to 4 weeks. The sampling rates of microcystins for two different exposure scenarios were derived (e.g., MC-LR: R (s) = 0.017 L/day under static and 0.087 L/d under turbulent conditions). R (s) values were further used for calculations of time-weighted average concentrations in natural water. Improved sensitivity and selectivity of the in-house-made sampler was observed in comparison with the commercially available Polar Organic Compound Integrative Sampler (POCIS). Comparisons of grab and passive sampling methods were performed during cyanobacterial water bloom season in the Brno reservoir, Czech Republic in 2008. Data obtained by passive sampling provided a more relevant picture of the situation and enabled better assessment of potential risks. The present study demonstrated that the modification of POCIS is suitable for monitoring of occurrence and retrospective estimations of microcystin water concentrations, especially with respect to the control of drinking water quality.