The aim of this research was to assess biochemical changes in red microalga Porphyridium cruentum biomass upon contact with silver nanoparticles obtained by biological synthesis, ranging from 12 - 30 nm in size. The essential biochemical parameters of biomass (proteins, lipids, carbohydrates and phycobiliproteins) and its antioxidant activity were determined. In this experiment, Porphyridium cruentum biomass collected at the end of the exponential growth phase was used. It was coupled with stable biomass yield. On average, Porphyridium cruentum dry biomass contains 30 - 35% crude protein. In the current study, the control sample of microalgal biomass contained 29% protein. Treatment of algal biomass for 24 hours with silver nanoparticles (AgNPs) resulted in severe protein degradation reaching 12%. There was no change in protein content into biomass after 48 hours of exposure to AgNPs. The content of phycobiliproteins in algal biomass also decreased drastically. Upon contact with AgNPs, the total content of phycobiliproteins in Porphyridium biomass after 24 hours decreased by 46%. Thus, when microalgal biomass came into contact with AgNPs within 24 hours, the content of phycoerythrin decreased by 40%, phycocyanin by 57%, and allophycocyanin by 40%. Moreover, subsequent exposure of biomass for 48 hours led to greater loss of phycobiliproteins, the amount of phycoerythrin was reduced by 92%, phycocyanin - by 97%, and that of allophycocyanin - by 94%. Microalga Porphyridium cruentum can produce rich amounts of polysaccharides with structural function and exopolysaccharides that offer a mechanical protection enabling the cells to develop in a range of environments. The results demonstrated that these compounds degraded rapidly during exposure to nanoparticles. In the first 24 hours of contact, the amount of polysaccharides decreased from 41.6% per dry biomass to 33.6%, and their quantity was 28.3% after 48 hours. Lipids produced by Porphyridium cruentum act as structural components of cell membranes, and maintaining their quantitative stability is a condition to successfully keep its cellular integrity. In the first 24 hours of biomass exposure to AgNPs, the amount of lipids decreased sharply - by 57% compared to the control sample. Under conditions of prolonged exposure to silver nanoparticles, the lipid content in algal biomass decreased to 65% compared to control sample. The values of ABTS assay showed a significant decrease in antioxidant activity of the aqueous extract obtained from Porphyridium biomass compared to the ethanol extract. In the first 24 hours of contact with AgNPs, the antioxidant activity of water extract decreased by 66%, and in the ethanol extract - by 52%. After 48 hours of algal biomass exposure to AgNPs, the estimation of antioxidant activity of both types of extracts in ABTS assay resulted in a slower decrease compared to the first 24 hours. Therefore, biochemical tests on the main parameters of Porphyridium cruentum biomass demonstrated the presence of a toxic effect, which manifested in the first hours of contact of algal biomass with silver nanoparticles obtained by biological synthesis involving an organic coating. One of the causative factors of the toxicity of silver nanoparticles to Porphyridium cruentum may be the variation in their size. In this case, such a factor as culture age did not mitigate the toxic effect of the nanoparticles.