Pollution of aquatic ecosystems by plastics with a particle sizes < 5 mm, or microplastics (MP), is becoming a global problem. MP possesses a threat to aquatic organisms, but also to other organisms due to the possibility of transmission through trophic levels. In this paper, toxicological studies of the effect of MP on three organisms were performed in order to determine the harmful effect of MP on them. Microalgae Chlorella sp. was exposed to different concentrations and particle sizes of polyethylene (PE), polypropylene (PP), polystyrene (PS), poly(vinyl-chloride) (PVC) and poly(ethylene-terephthalate) (PET). The bacterium Vibrio fischeri and zebrafish (Danio rerio) embryos were exposed to different concentrations and sizes of PS. Chlorella sp. was exposed to particle sizes of 500 – 1000 and >1000 µm and concentrations of 10; 25; 50 and 100 mg/L. The greatest inhibition of growth of Chlorella sp. (5.01 %) was caused by PS particle sizes >1000 µm and concentrations of 100 mg/L, respectively. Due to the not so significant difference between the values of algae growth inhibition for the tested MP particle sizes, in the second part of the experiment the algae was exposed to smaller particle sizes <300; 300 – 500 and 600 – 710 µm and concentrations 10; 500 and 1000 mg/L. The influence of process parameters (average particle sizes and MP concentrations) on the toxic effect of MP on freshwater algae was investigated. Using a combination of the full factorial plan and the response surface method (RSM), conditions were determined in which the maximum inhibition of algal growth during exposure to PE (8.62 %) was determined at a concentration of 500 mg/L and particle sizes <300 μm. According to the obtained results, it can be concluded that the concentration of particles has a greater influence on the inhibition of algae than the sizes of the particles. Vibrio fischeri was exposed to PS particle sizes of 0.43 and 0.25 μm and concentrations from 0.0001 to 1000 mg / L, and inhibition of luminescence was monitored. Inhibition increased with decreasing particle sizes and increasing concentrations. Zebrafish embryos were exposed to PS sizes of 0.43; 0.25 and 0.12 µm and concentrations between 0.0001 and 10 g/L. The first effect on embryos was caused by particle size of 0.12 µm and a concentration of 0.1 g/L. This particle size also caused the first 100 % mortality of the embryos at a concentration of 6 g/L. It was concluded that the toxic effect on the tested organisms depends on the size, concentration and type of MP.