Silver and gold nanoparticles are a promising tool for medical and industrial application, therefore, their ecotoxicity should be carefully examined. The effect of silver (12 nm) and gold (4.7) nanoparticles coated with polyethylene glycol on Spirulina platensis biomass growth and biochemical composition was investigated. The spirulina cultivation medium was supplemented with nanoparticles in concentration range 0.025-0.5 µM. Given concentrations stimulated spirulina biomass growth, while content of proteins, carbohydrates and auxiliary pigments was affected insignificantly by presence of nanoparticles in cultivation medium. The pronounced effect of gold nanoparticles at concentration 0.5 µM on the lipids content was observed. TEM images demonstrate that nanoparticles penetrate inside the cell, resulting in ultrastructural changes in cells.