Plastic pollution, which has been on the rise in recent years, is attracting increasing attention from both scientists and the general public. As a cheap, light, strong, durable and resistant material, plastic has a wide application in all industries and despite its recyclability, it ends up in the environment as a waste with its anthropogenic effect. Under the influence of weather conditions, it decomposes and fragments into smaller pieces, into plastic particles smaller than 5 mm, which we call microplastics, and poses a danger to both flora and fauna and the human world. Plastic production processes involve the use of a wide range of chemicals (e.g., initiators, catalysts, flame retardants, stabilizers, plasticizers, fillers, and dyes). Due to the use of a large proportion of additives, there is a potential migration of these harmful additives through today's ubiquitous microplastics into the environment and, due to that, there occur side effects on humans and other organisms exposed to it. Microalgae Scenedesmus sp. was used to monitor the toxicity of mictoplastics polyethylene terephthalate (PET). The effect of three particle sizes was investigated: 100 – 300 μm, 300 – 500 μm and 500 – 700 μm at five different concentrations of PET microplastics: 50 mg/L, 250 mg/L, 500 mg/L, 750 mg/L and 1000 mg/L for 72 hours. According to the results, it can be seen that different concentrations and particle sizes of PET have different effects on the microalgae, but the number of microalgae cells in the samples and control did not differ too much, that is no greater reductions in the number of microalgae cells compared to the initial number have been recorded.