According to Plastics Europe, total global production of plastics has risen from 1.5 million tons in the year 1950 to 245 million tons in the year 2008. Simultaneously, the amount of plastics littering terrestrial and marine habitats has risen proportionally and is bound to rise with the continual increment in worldwide plastic production and consumption. While plastics have bestowed the global population with copious societal benefits, they display a variety of adverse repercussions as mega-, macro-, meso-, micro- and even nano- debris in the natural environment. In the past 50 years, plastics have become a pervasive and abundant pollutant of the terrestrial and marine biosphere, subsisting in all sizes, shapes and colors, thus being available to a wide range of species, particularly to non-selective foragers. In the marine environment, the effects of macro-plastic debris is well documented while research on the effects of microplastics is fairly recent, revealing ever more adverse impacts with increasing investigation. However, effects on the organismic level of biological organization are only understood rudimentarily but are essential for inference of the integral effects on higher levels of biological organization, such as on populations and ecosystems. In a new approach implementing cellular biomarkers of early toxic effects in combination with a novel method for microplastic detection in frozen tissue sections using polarized light microscopy, blue mussels Mytilus edulis (L.) were exposed to a constant concentration of the model microplastic High-Density-Polyethylene (HDPE) powder consisting of particles ranging 0 – 80 μm in size (Gaussian distribution) for a series of time levels ranging from 0 h to 96 h. To examine whether microplastics are assimilated by invertebrates and to analyze the potential histological effects upon ingestion, mussel health status was determined by measuring i) the condition index and ii) lysosomal membrane stability and by semiquantitatively assessing iii) the degree of neutral lipid accumulation, iv) the degree of particles accumulated in vacuoles as well as histopathological lesions described as the degree of v) granulocytoma formation and vi) vacuolization of digestive gland tubules in the mussel digestive gland. The strongest effects on mussel health status were evident in the incidence of granulocytoma formation, which occurred as early as after 12 h of exposure and significantly correlated with the degree of particle encapsulation in vacuoles. Particles were already accumulated in vacuoles after 3 h of microplastic exposure and remained consistently high. Significant effects were also apparent in lysosomal membrane stability peaks, which all exhibited significantly reduced destabilization times after 96 h and one peak after 12 h as well (peak 2). In contrast, vacuolization was high, both in control and treated mussels and no effects were detectable with respect to the condition index and neutral lipids accumulation, which both represent long-term biomarkers of effect. The study corroborates the potential hazardous effects of microplastic exposure and proved that the newly developed method of microplastic detection with polarized light microscopy in combination with biomarkers of effect is a suitable approach for the evaluation of potential hazardous effects of microplastics at the organismic level. Keywords: Microplastics; Lysosomal membrane stability; Marine pollution; Marine litter